shell valves genera species valve muscles animal appendages surface attached
THE BRACHIOPODA constitute an important and well-defined class of Invertebrates, but the exact position the group should occupy in that division of the animal kingdom is still a matter upon which anatomists have not entirely agreed. For many years the species composing the class were referred to the genus Anemia of the Lamellibranchiata, but, as was judiciously observed by Edward Forbes, " a close examination shows that there is no relationship between them, but only a resemblance through formal analogy." Milne-Edwards separated the Mollusca, into two great divisions, Mollusca and Molluscoida, and in the last he placed the Brachiopoda, Polyzoa, and Tunicata, an arrangement that has been followed by many naturalists. Although the greater number of zoologists have admitted the close connection existing between the Polyzoa and Brachiopoda, considerable doubt has been expressed with respect to the affinities and position of the latter in relatiom to the Tunicata ; moreover, a strenuous effort has been made within the last few years by Steenstrup, Morse, Kowalevsky, A. Agassiz, and others, to demonstrate that the affinities of the Brachiopoda and Polyzoa are with the Worms, and that they should form classes of Annulosa, and be placed close to the Annelids.
According to Agassiz, the transition between such types as Pedicellina to Menibranipora and other incrusting Polyzoa is readily explained from the embryology of Thecidium, and, in fact, all incrusting Polyzoa are only communities of Brachiopods, the valves of which are continuous and soldered together, the fiat valve forming a united floor, while the convex valve does not cover the ventral one, but leaves an opening more or less ornamented for the extension of the lophore. Both Gratiolet and Hancock have expressed the opinion that the Tunicata are in no way related to the Brachiopoda, and that we cannot place these last and 'the Polyzoa along withthe Tunicata in the same division. Gratiolet and sonic others have considered the Brachiopoda to be allied to the Crustacoa, while even the asteridian affinities of the class have been hinted at by King.''" No doubt can be entertained, after perusing the admirable memoirs by Morse and Kowalevsky on the embryology of Terebratala, Terebratulina, Argiope, and Thecidium, that the genera composing the class and Amphetrite possess many important features in common, but almost any invertebrate groups might be annelidilized by overrating certain points in their affinities. Mr Dall thinks that the general conclusion with reference to the affinities of the Brachiopoda will be something like this. There is much reason for supposino-b that all the Molluscs and Molluscoids came from the stock out of which the Worms have developed. Indeed, as Huxley has said, they are only isomerous Worms with many special modifications. It is natural, therefore, that the oldest and lowest forms should retain many of the characteristics of the oldest and most simple Worms, especially those which have been modified by a tubular habit. But, on the whole, the modifications are so important that we may continue to consider (if in the specializing tendency of present study we can retain any general divisions of Invertebrates) that the Molluscoids and Molluscs do form two groups somewhat aside froth others, and somewhat more nearly related to each other than to the divisions external to them. Therefore, although it may turn out that the Brachiopoda constitute a class close to the Annelids, it cannot be denied that they possess many molluscan characters that cannot be overlooked, and are, under any circumstances, entitled by their importance and numerous Pleuropygia (Bronn, 1862), Inarticulata (Huxley, 1869), Lyopomata (Gill, 1871), have been made use of ; while for the second division, comprising Terebratula, Rhynchonella, &c., the names Athropomata (Owen, 1858), Apygia (Bronn, 1862), Articulata (Huxley, 1869), Arthrodistinctive features to constitute a well-defined and separate class.
The name BRACHIOPOD (13paxirov, an arm, 7ras, 77-08k Name. a foot) was proposed for the class by Cuvier in 1805, and by Dumeril in 1809, and has since been very extensively adopted. Blainville in 1824 proposed as a substitute for the Cuvierian name that of Palliobranehiata (pa/4q/m, a mantle ; bran•hia?, gills), on account of the respiratory system being combined with ,the mantle on which the vascular ramifications are distributed. Prof. King has always adopted the latter name, and perhaps rightly objects to Cuvier's ou the ground that it is a misnomer, for the two variously curved and cirrated brachial or labial appendages, improperly designated as arms or feet, were subsequently found not to subserve the function of locomotive organs.
Before describing the various parts of the animal and Two diviits shell, it may be as well to mention that it had been sions. many times suggested by Owen, Bronn, Huxley, Gill, and others, that the class could be advantageously divided into two primary groups. Thus, for the first division, including Liniula, Discina, the names Lyopomata (Owen, 1858), psnnata (Gill, 1871) have been proposed. Prof. King, considering these names to be objectionable, and in some cases inadmissible on certain grounds, in 1873 substituted the name Tretentecata for the first group, the intestine being provided with an anal aperture, while the second group, to which he gives the name of Clistenterota, would embrace animals that are destitute of that organ ; but it must also be remembered that the presence or absence of an anal aperture has been ascertained in only three or four recent genera, and that we are compelled, in a measure, to take for granted that what we find to be the case in Lingula and Terebratula is also so in the many extinct families and genera of which the animal cannot be examined.
With the character above enounced, we generally find structural modifications of the hinge and other differences in the animal, and especially so in what relates to the muscular system. In the opinion of Prof. King the absence of an anal vent in the Olistenterata makes them inferior to the aniferous Tretenterates.
The animal of the Brachiopod is in all cases protected by a shell composed of two distinct valves ; these valves are always, except in cases of malformation, equal-sided, but not equivalved. The valves are, consequently, essentially symmetrical, which is not the case with the Lamellibranchiata or Conchifera, - so much so, that certain Brachiopod shells received the name Lampades, or lamp shells, by seine early naturalists ; but while such may bear a kind of resemblance to an antique Etruscan lamp, by far the larger number in no way resemble one. The shell is likewise most beautiful in its endless shapes and variations. In some species it is thin, semi-transparent, and glassy, in others massive. Generally the shell is from a quarter of an inch to about four inches iu size, but in certain species it attains nearly a foot in breadth by something less in length, as is the case with Productus giganteus. The valves are also in some species very unequal iu their respective thickness, as may be seen in Productus Llangollensis, Davidsonia 1 erueecilii, &e., and while the space allotted to the animal is very great in many species, as in Terebratula splice•oidalis, it is very small in others belonging to ,Stropkontena, Leptcena, Chonetes, &c. The ventral valve is usually the thickest, and in some forms is six or seven times as great as the opposite one. The outer surface of many of the species presents likewise the most exquisite sculpture, heightened by brilliant shades, or spots of green, red, yellow, and bluish black. Traces of the original colour have also been preserved in some of the fossil forms ; radiating bands of a reddish tint have been often seen in well-preserved examples of Terebratula latstata, T. saculys, T. comanunis, biplicata, and of several others. Some specimens of T cornea are of a beautiful pale pink colour when first removed from their matrix, and E. Deslongchamps has described the tint of several Jurassic species.
The valves have been distinguished by various names, but those of dorsal and ventral are in most general use. The ventral valve is usually the largest, and in many genera, such as Terebratula and lellynchonclla, has a prominent beak, with a circular or otherwise shaped perforation or foramen at or near its extremity, partly completed by one or two plates, termed a deltidium. Through the foramen passes a bundle of muscular fibres, termed a peduncle, by which the anneal • is in many species attached to submarine objects during at least a portion of its existence. Other forms show no indication of ever having been attached, while some that had been moored by means of a peduncle during the early portion of their existence have become detached at a more advanced stage of life, the opening becoming gradually cicatrized, as is so often seen in Strophontema rhontboida/is, Oraceina anomala, &c. Lastly, sonic species have adhered to submarine objects by a larger or smaller portion of their ventral valve, as is the case with many forms of Crania, Thecidium, Davidsonia, &c. Some Cranias are always attached by the whole surface of their lower or ventral valve, which models itself and fills up all the projections or depressions existing either on the rock, shell, or coral to which it adhered. These irregularities are likewise, at times, reproduced on the upper or dorsal valve. Some species of Strophatosia and Productus seem also to have been moored during life to the sandy or muddy bottoms on which they lived, by the means of tubular spines of greater or lesser length. The interior of the shell varies very much according to families and genera. On the inner surface of both valves several well-defined muscular vascular and ovarian impressions are observable ; they form either indentations of greater or lesser size and depth, or occur as variously shaped projections. In the Trinterellidce, for example, some of the muscles are attached to a massive or vaulted platform situated in the medio-longitudinal region of the posterior half or umbonal portion of both valves. In addition to these, there exists in the interior of the dorsal valve of some genera a variously modified, thin, calcified, ribbon-shaped lamina or skeleton for the support of time labial or brachial appendages ; and so varied, yet constant in shape to certain species is this laminal apophysis, that it has served as one of the chief characters in the creation of both recent and extinct genera. The apophysis is more or less developed in some genera than in others. In certain forms, as in Terebratida and Terebratutina' it is short and simple, and attached to a small divided hinge- plate, the two riband-shaped lamina being bent upwards in the middle (fig. 15). The cardinal process is prominent, and on each side of the binge-plate are situated the dental sockets ; the loop in Terebratidina becomes annular in the adult by the union of the oral processes or crura (fig. 16). In Waldheimia it is elongated and reflected ; the hinge-plate large, with four depressions, under which originates a medal septum, which extends more or less into the interior of the shell (figs. 13 and 14). In Terebratella the loop is attached to the hinge-plate and to the septum (fig. 17). In .21/eger/ia it is three times attached, first to the hinge-plate, and then to the septum by processes from the diverging and reflected positions of the loop. In May as the apophysary system is composed of an elevated longitudinal septum reaching from one valve to the other, to which are affixed two pairs of calcareous lamelloc, the lower ones riband-shaped ; attached first to the hinge-plate, they afterwards proceed by a gentle curve near to the anterior portion of the septum, to the sides of which they are affixed ; the second pair originate on both sides of the upper edge of the septum, extending in the form of two triangular anchor-shaped lamely (fig. 18). In Bouchardia the septum only is furnished with two short anchor-shaped lamely. Many more modifications are observable in different groups of which the great family Terebratulidco is composed, and which will be found fully described in Davidson's and other authors' works on the Brachiopoda. In Thecidium (figs. 3, 4) the interior of the dorsal valve is variously furrowed to receive a testaceous ridge folded in two or more lobes. In the family Spiriferidce there are two conical spires directed outwards, and nearly filling the cavity of the shell (fig. 5) ; while in Ater -y-pa the broad spirally coiled lamely are vertical, and directed towards the centre of the valve. In the Rhynchonellic/ce there are two short slender curved lamina', while in many genera and even families, such as the Productidce, Strophomenicke, Lingulidce, Diseinidce &c., there exists no calcified support for the labial appendages. The ventral valve in many of the genera is provided with two curved hinge-teeth, which fit into corresponding sockets in the opposite valve, so that the valves cannot be separated without breaking one of the teeth. Nearly all the genera composing the division Clistenterata have their valves articulated, while those forming the Tretenterata have theirs kept in position by the means of muscles especially adapted to that purpose ; but in one of the most natural groups, viz., that of the Productida?, we find genera presenting both conditions.
The intimate structure of the shell has been minutely investigated by Dr Carpenter, Prof. King, Dr Gratiolet, and several others, and been found to be distinct from that of the Lamellibranchiata, and Gasteropoda. Dr Carpenter informs us that there is not in the shell of the Brachiopoda that distinction between outer and inner layers, either in structure or mode of growth, which prevails among the ordinary bivalves ; that it seems obvious, both from the nature of the shell substance and from the mode in which it is extended, that the whole thickness of the Brachiopod shell corresponds with the outer layer only of the Lamellibranchiata ; and that he has occasionally met with a second layer in recent Terebratuhe, within the earlier portion of the shell, but confined to only a part of the surface instead of extending beyond it. In some families composing the Clistenterata it consists, according to Prof. King, of three divisions, the innermost and middle ones, which constitute the entire thickness of the valve, being calcareous with a prismatic or fibrous structure, while the outer divisions would consist of a very thin mombranc. The innermost and intermediate divisions are in some families traversed by minute tubular canals, which pass from one surface to the other, for the most part in a vertical direction, and at tolerably regular intervals, but just before terminating near the outer surface of the epidermis their orifices suddenly become dilated, the lower half of the canals being often considerably smaller in diameter than the upper half. The canals are occupied by meal processes proceeding from the mantle or the fleshy covering of the animal. Their function is, according to Dr Carpenter, branchial or subservient to respiration ; but if there exists an outer epidermis, as described by King, which covers their expanded terminations, there would be no communication between the surrounding sea water and the mantle. In the Rhynchonellidce and in some other families the shell structure would, according to Dr Carpenter, consist of flattened prisms of considerable length, arranged parallel to each other with great regularity, and obliquely to the surface of the shell, the interior of which is imbricated by their outcrop. In certain genera, such as _Uvula and Discina, no canals traverse the shell from the inner to the outer surface. The shell structure, according to Dr Gratiolet, would consist of two distinct elements, that is to say, a corneous or horny animal substance, and a testaceous one; these occur in alternate layers of unequal thickness. The testaceous layers recall the structure observable in the Terebratulithr, being traversed by numerous canals of extreme or microscopic minuteness. As Mr Woodward observes in his excellent manual of the Mollusca, Prof. Huxley has suggested that the caeca are analogous to the vascular processes by which in many Ascidians the tunic adheres to the test, the extent of which adhesion varies in closely allied genera. It seems, however, strange that these tubular perforations should not have been essential to the species of every family composing the class if they are really subservient to respiration. The subject will therefore demand further consideration.
The anatomy of the Brachiopoda has been the subject of elaborate investigations by Cuvier, Vogt, Huxley, Hancock, Gratiolet, ‘Voodward, Deslongehamps, King, and others, while of late years much light has been likewise thrown on the embryology and early stages of the groups by Steenstrup, Lacaze-Duthiers, Morse, P. Miiller, Oscar Schmidt, M'Crady, Kowalevsky, and others. Some differences in opinion, it is true, have been and still are entertained with respect to the exact function to be attributed to certain parts of the animal, but on all essential questions there is a pretty general agreement.
According to Morse the Brachiopoda are reproduced by eggs, generally kidney-shaped and irregular, which are discharged from the anterior margin of the. shell, and drop just beyond the pallial membrane, hanging in dusters from the setae. Some uncertainty has prevailed as to whether there is a male and female individual. LacazeDuthiers and Morse state that the sexes are separate, and describe them as such in Thecidium and Terebratulina, and the French zoologist goes so far as to suggest that a difference is even observable in the shell, but the statement requires verification. Prof. Morse describes the embryo of Terebratulina l‘ith great minuteness during its six stages of development. It is divided into two, three, or four lobes clothed with vibratile cilia, and before becoming attached swims or whirls head foremost by means of vibratile cilia which cover the body. The same distinguished American zoologist describes with equal care the formation of the shell from its first stage of development to the adult condition. Lacaze-Dutliiers alludes to two and four eye spots in the embryo of Thecidium, and states that the animal appears to be in some measure sensible to light.
The mouth conducts by a narrow oesophagus to a simple stomach which is surrounded by a large granulated liver. Owen's " hearts " have been found to be oviducts, while the true heart consists of a p3-riform vesicle appended to the dorsal surface of the stomach. The digestive organs and viscera, as well as the muscles, which take up only a small place in the neighbourhood of the beak, are separated from the great anterior cavity, and protected by a strong membrane in the centre of which the mouth is situated. The nervous system consists of a principal ganglion of no great size.
Both valves are lined by a delicate membrane termed the pallium or mantle ; it secretes the shell, and is generally fringed with horny bristles or setm. It is composed of an outer and inner layer, between which are situated the blood channels or lazunes ; in fact, all the internal parts of the shell are lined by the inner layer of the mantle, with the exception of the spots where the muscles attach themselves to the shell. The outer layer lines closely the inner surface of the valves to which it adheres, and in those species in which the shell is traversed by canals there exist, on the surface of the mantle facing the inner surface of the valves, corresponding short cylindrical membranous projections or lacunes, which insert themselves into the small tubular orifices that traverse the shell. The cEecal prolongations do not exist in those genera, such as Rhyne/tom/la, where the shell is deprived of tubular perforations. The inner layer is rather thicker than the opposite one, and is covered with vibratile cilia. As stated by Nicholson and other anatomists, the blood channels form a remarkable system of more or less branched tubes, anastomosing with one another, and ending in the meal extremities. This, which has been termed by Huxley the arterial system, communicates with the perivisceral cavity by means of two or four organs, which are called pseudo-hearts, and which were at one time supposed to be true hearts. Each pseudo-heart is divided into a narrow, elongated external portion (the so•called "ventricle"), which communicates, as Hancock has proved, by a small apicial aperture with the pallial cavity ; and a broad, funnel-shaped so-called " auricle," communicating on the one hand by a constricted neck with the so-called " ventricle," and on the other by a wide patent month with a chamber which occupies most of the cavity of the body proper, and sends more or less branched diverticula into the pallial lobes (Huxley). The channels vary in their dispositions and details in different genera, and as they project to some small extent, leave corresponding indentations on the inner surface of the shell, so that their shape and directions can very often be traced on fossil and extinct genera as well as if the animal was still in life ; this may be seen in the numerous illustrations appended to Davidson's and other authors' works treating of fossil Brachiopoda. There are usually four principal arterial trunks in each lobe of the mantle; the two central ones run direct to the front, near to which they bifurcate, while the outer ones give off at intervals on the side facing the lateral margin of the valves a series of branches which bifurcate several times, It has been observed by Hancock. that tire inner lamina of the mantle, and more particularly that portion of it forming the floor of the great pallial sinuses, will undoubtedly assist in purifying the blood. In 165-t, in his review of Davidson's great work on British fossil I lrachiopoda, Oscar Schmidt called attention to an important anatomical omission, 'namely, the existence of a vast number of microscopic, flattened, calcareous, denticulated plates or spicuhe on certain parts of the surface of the mantle, and destined, no doubt, to stiffen and protect the portions that contain them ; and it was, moreover, observed by Hancock, and afterwards by Deslongehamps, that these calcareous plates are not to be found equally distributed over all the surface of the mantle, but only in those portions in connection with the great lacnnes or veins, the labial appendages, and the perivisceral cavity. These spicuke do not appear to be present in every species, and are totally absent in Ldiquia, Rhynchond/a, and others. Deslongchamps observes that if we examine the genera Kraussina, Terebratula, Tercbratulina, llfegerlia, and Platylia, we have a series wherein the number and consistence of the calcareous portions increase in a very rapid manner, and that the spicules lie over each other several times, leading the observers by insensible degrees to Thecidium, in which the spicula., are soldered together, and incrust the mantle to such an extent that it is no longer distinct from the shell itself.
The brachial appendages are a pair of singular organs eminently characteristic of the Brachiopoda ; they often are more correctly termed labial appendages on account of each member being a prolongation of the lateral portion of the lips or margin of the mouth. The Lamellibranchs or Conchifera have analogous appendages, but very much less developed. They assume different shapes in different genera, and are supported, or otherwise, by the more or less complicated skeleton already described. The labial appendages, whatever may be the shape and convolutions they may assume, fill the larger portion of the cavity of the shell in front of the visceral chamber ; they are formed of a Meinbranous tube, fringed on one side with long flexible cirri, and occupy almost the whole of the pallial cavity, but were not capable of being protruded in those families in which they were folded back upon themselves and supported by a calcareous skeleton, as in Traldheimia, Terebmtella, he. Barrett, who has examined the animal of Terebratulina mind seiTentis in life, states that it showed more of itself than Wahl. heimia cranium, which might be supposed from the labial appendages being in the first very slightly supported by a small loop. Barrett observed, likewise, that it protrudes its cirri further, that the cirri on the reflected part of the brachial appendage are shorter than on the first part, mid were almost constantly in motion, and often seen to convey small particles to the channel at their base, and that the cirri are bent up when the brachial appendages are retracted, but are generally uncoiled and straightened when the shell is opened, before which the animal has often been observed to protrude a few of its cirri, and move them about to ascertain if any danger threatened. In Rhynchonella, where the elongated spiral labial appendages are slightly supported only at their origin by two short calcareous processes, they can at the will of the animal, according to Owen and Morse, be unrolled and protruded to some distance beyond the margins of the valves, and when forcibly stretched out they are said to be more than four times the length of the shell, and to support conic 3000 cirri. We may mention, likewise, that Otto Fredericic Miler, having dredged from the Lake of Droeback, in Norway, a number of Terebratulce (probably belonging to Rhynchonella psittacea), and placed them in a glass of water, he observed that they gracefully extended their spirally coiled brachial appendages. It must, however, remain for ever uncertain whether, in the extinct genera Spirifent, Atrypa, and others, in which the spirally coiled fleshy labial appendages were supported throughout their entire length by a calcified skeleton, the animal could protrude them beyond the margin of the valves. In some families - 111tynchonellidw, Producticloe, and others - these organs are spiral and separate; in many the separation is only at their extremities. There can be very little doubt that these elegant organs, provided as they are with cirri and cilia, are not only instrumental in conveying microscopic organisms to the month, which, as seen in fig. 1 9, is situated between the appendages at their origin, but are likewise subservient to the functions of respiration. Hancock observes that to prove that the brachial organs subserve the function of gills, as well as that of sustentation, it is only necessary to refer to the manner in which the blood circles round the labial appendages and is carried to the cirri, but more particularly to its circulating through these latter organs, and returning direct from them to the heart.
As the number and position of the muscles differ materially- in the two great divisions into which the Braehiopoda have been grouped, and to some extent also in the different genera of which each division is composed, it may be desirable to treat this subject under two separate heads. Unfortunately almost every anatomist who has written on the muscles of the Brachiopoda has proposed different names for each muscle, and the confusion thence arising is much to be regretted. In the Clistentcrata, of which the genus Terebratula may be taken as an example, five or six pairs of muscles are stated by Hancock, Gratiolet, and others, to be connected with the opening and closing of the valves, or with their attachment to or movements upon the peduncle. First of all, the adductors or occlusors consist of two muscles, which, bifurcating near the centre of the shell cavity, produce a large quadruple impression on the internal surface of the small valve (fig. 13, a, a'), and a single divided one towards the centre of the large or ventral valve (fig. 12, a). The function of this pair of muscles is the closing of the valves. Gratiolet, who has likewise described with great minuteness the muscles of the Brachiopoda, informs us that those which close and open the valves were the only ones known to Pallas, but that he defined their position and functions clearly. The same was done by Blainville and Quenstedt, but the absence of good figures caused much uncertainty to prevail. This deficiency was subsequently supplied by Hancock and Gratiolet's admirable illustrations. Two other pairs have been termed divaricators by Hancock, or cardinal muscles (" muscles diducteurs " of Gratiolet), and have for function the opening of the valves. The divaricators proper aro stated by Hancock to arise from the ventral valve, one on each side, a little in advance of and close to the adductors, and after rapidly diminishing in size become attached to the cardinal process, a space or prominence between the sockets in the dorsal valve. The accessory divaricators are, according to the same authority, a pair of small muscles which have their ends attached to the ventral valve, one on each side of the median line, a little behind the united basis of the adductors, and again to the extreme point of the cardinal process. Two pairs of muscles, apparently connected with the peduncle and its limited movements, have been minutely described by Hancock as having one of their extremities attached to this organ. The dorsal adjustors are fixed to the ventral surface of the peduncle, and arc again inserted into the hinge-plate in the smaller valve. The ventral adjustors are considered to pass from the inner extremity of the peduncle, and to become attached by one pair of their extremities to the ventral valve, one on each side of and a little behind the expanded base of the divaricators. The function of these muscles, according to the same authority, is not only that of erecting the shell, they serve also to attach the peduncle to the shell, and thus effect the steadying of it upon the peduncle. Gratiolet describes the peduncle with great care, and states it to be composed of two portions - 1 st, of a horny sheath formed of concentric epidermic layers, very analogous to that which Vogt has described in Linyula ; and 2d, a fibrous stem enveloped by the sheath. This stein, composed of tendinous fibres, is fixed by its free extremity to different submarine objects ; the other extremity passes through the foramen, and is ended by a bulbous projection.
Such is the general arrangement of the shell muscles in the division composing the articulated Brachiopoda, making allowance for certain unimportant modifications observable in the animals composing the different families and genera thereof. Owing to the strong and tight interlocking of the valves by the means of curved teeth and sockets, many species of Brachiopoda could open their valves but slightly. In some species, such as Thecidium, the animal could raise its dorsal valve at right angles to the plane of the ventral one (fig. 4).
In the Tretenterata, of which Lingula and Discina may be quoted as examples, the myology is much more complicated, and anatomists have differed considerably in their respective views concerning the function of some of the muscles. They have been carefully described by Owen, Vogt, Hancock, Gratiolet Woodward, and others, and more recently by King, whose views seem to carry with them a greater degree of plausibility. Of the shell or valvular muscles he makes out five pairs and an odd one, and individualizes their respective functions as follows : - Three pairs are later«1,having their members limited to the sides of the shell ; one pair are transmedians, each member passing across the middle of the reverse side of the shell, while the odd muscle occupies the um-banal cavity. The central and umbonal muscles effect the direct opening and closing of the shell, the laterals enable the valves to move forward and backward on each other, and the transmediams allow the similar extremities (the rostral) of the valves to turn from each other to the right or the left on an axis sabcentrically situated, that is, the medio-transverse region of the dorsal valve. It was long a matter in discussion whether the animal could displace its valves sideways when about to open its shell, but this has been actually observed by Professors Scamper and Morse, who saw the animal perform the operation. They mention that it is never done suddenly or by jerks, as the valves at are first always pushed to one side several times and back again on each other, at the same time opening gradually in the transverse direction till they rest opposite to one another and widely apart. Those who have not seen the animal in life, or who did not believe in the possibility of the valves crossing each other with a slight obliquity, would not consent to appropriating any of its muscles to that purpose, and consequently attributed to all the lateral muscles the simple function of keeping the valves in an opposite position, or holding them adjusted. We have not only the observations of Sniper and Morse, but the anatomical investigations of King, to confirm the sliding action or lateral divarication of the valves of Ling?thr.
In the Clistenterata, where no such sliding action of the valves was necessary or possible, no muscles for such au object were required, consequently none took rise from the lateral portions of the valves as in lingula ; but in an extinct group, the Trimerellida,, which seem to be somewhat intermediate in character between the Tretenterata and Clistenterata, have been found certain scars, which appear to have been produced by rudimentary lateral muscles, but it is doubtful (considering the shells are furnished with teeth, though but rudely developed), whether such muscles enabled the valves, as in Lingula, to move forward and backward upon each other. We do not yet possess any reliable observations as to the manner in which 1) iscina opens its valves, but Mr Barrett, who observed Crania in life, informs us that the valve opens by moving upon the straight hinge, without sliding the valve. There are muscles connected with other portions of the animal in both groups, such as the parietal muscles, strongly defined in the Tretenterata, and distinctive peculiarities of the peduncle, hut the limited space at our disposal will not admit of entering upon further anatomical details.
The Brachiopocla all inhabit the sea, and are supposed to have attained their full growth in a single season. A. vast amount of important and accurate information has been collected during the past ten years with respect to the geographical distribution of recent species, as well as to the marine depths they inhabit or prefer. This important knowledge is mainly due to the numerous well conducted and equipped dredging expeditions carried on by private individuals and by the Governments of the leading maritime states. It would not be possible to give here the names of all those naturalists who have contributed to this portion of our information, but we cannot pass over those of Edward Forbes, J. G. Jeffreys, W. B. Carpenter, W. H. Dall, W. Thomson, E. Suess, A. Adams, H. Coming, &c. Previous to these investigations the data we possessed with respect to the habitat and ranges of depth were in most cases vague and unsatisfactory. It has been ascertained that the Braclnopoda are much localized, and usually occur iu great numbers in their favourite haunts. Jeffreys does not believe that the habitat of any Invertebrate animal is affected by bathymetrical conditions, and that the same species will occur at various depths. We can say nothing certain with respect to the ranges of depth at which the extinct species lived, but some idea as to their probable depths can be surmised from a study of the recent species. As far as our present information will carry us the Tretenterata do not appear to have been found at a greater depth than 1360 to 2000 fathoms. Litigukt abounds in particular haunts at about half the tide-mark, and partly buried in mud, or at depths varying from three or four inches from the surface of the sea to seventeen fathoms. Prof. Morse describes a species which he found in vast numbers in a sand shoal at low water ; the peduncle, six times the length of the shell, was partly encased in a sand tube (fig. 9). Ile observed likewise that this species (Lingelu pyrantidata) had the power of moving over the sand by the sliding motion of the two valves, using at the same time the fringes of setae, which swing promptly back and forth like a galley of oars, leaving a peculiar tract in the sand. In the motion of the setae he noticed the impulse commencing from behind and running forward. Discina has been found attached to stones at low water mark, and dredged from depths ranging from 5 to nearly 2000 fathoms. They are very often clustered together in vast numbers, each adhering by its peduncle to the surface of the shell of its neighbour, one above the other, till they form a living mass of considerable breadth and thickness. Crania is found in great numbers adhering to stones and shells at depths of from 18 to 530 fathoms. The genera and species of the Clistentarata live at depths ranging from about half tide mark to that of 2600 fathoms. Terebratulina caput serpents's was found by the late R. T. Loweliviry attached to rocks at low water mark on a part of the Scottish coast, where the tide falls only a few feet, but the same species has been dredged alive from depths varying from 3 to upwards of 150 fathoms. Prof. Jukes got immense numbers of specimens of Traldlieimia flavescens or Australis while boating in Australia among the reefs. They were merely washed by the tide, and he gathered them with his hand like limpets on the shore. M. Velain picked up a small species of Kraussina in vast numbers on the shore in the interior crater of the island of St Paul, the shell being alternately covered with water and left dry at every tide. Kraussina rubra, from the coast of Natal in South Africa, was described by Dr Gray as having been found attached in great numbers to ascidia and stems of sea-weeds, and Mr Jeffreys had also previously noticed a small European species similarly fixed to sea-weeds. In general, however, it may be said that the larger number of species inhabit depths varying from 5 to 300 or 400 fathoms. Several species live attached to coral reefs. Waldheintia cranium has been obtained from depths varying from 160 to 228 fathoms. Barrett and Jeffreys state that Terebratulina caput seTentis manifested a remarkable power and disposition to move on its peduncle, and that it was incessantly opening and folding its brachial appendages, and drawing in and sucking in, by means of the whirlpool thus caused, every animalcule within its influence.
It is now necessary to say a few words with reference to the classification of the Brachiopoda, and in drawing up any scheme of arrangement due regard must be paid to the extinct forms, which vastly outnumber those of the present seas. • The first species belonging to the class were imperfectly and quaintly described as well as figured by Fabio Columna as far back as 1606. Since then so many paleontologists have contributed to the elucidation of the fossil species that it would not be possible to give all their names ; we must not, however, omit to record those of Linnmus, Defrance, Von Buch, Alcide D'Orbigny, De Blainville, Sowerby, Barrande, De Verneuil, Deslongchamps (father and son), De Koninek, E. Suess, W. King, F. M'Coy, J. Hall, Billings, Dolman, Dall, Fischer, Pander, C. Moore, Eichwald, Kutorga, Keyserling, Samlberger, Sequenza, Salter, Morris, Meek, and Davidson.
Various schemes of classification have been proposed, but none as yet can be said to be more than provisionally satisfactory, because before one can classify it is necessary to understand all the characters of the species one has to arrange in their more or less natural groups, and we are not yet in possession of all that necessary information.
In 18.53 Davidson divided the Brachiopods into eight families, comprising twenty-four genera and about as many sub-genera, but during the years that have elapsed from that to the present time, about seventy more genera and sub-genera have been described, so that as many as one hundred and twenty-three so-termed genera have now to be classed into their respective families, a task which has not yet been satisfactorily accomplished. It is, however, very probable that the number above given has been exaggerated, and that when our knowledge has increased, some of them will have to be placed among the synonyms.
It will be necessary in every scheme of classification to admit the two great divisions Tretenterata and Clistenterata.
The TRETENTERATA would comprise the families LinDiscinichr, Crailiada,, Trimerellidcr, and perhaps one or two others.
The CLISTENTERATA would include the families Tercbratnlidm, Thecida,the, Spiriferida', Z(hyndhonellilw, Pentameridce, Strophomerida,, Orthicke, Productida,, and perhaps two or three others that will have to be characterized. By far the larger number of described genera and species would find their place in this last great division and the above-named families. We will now very briefly notice some of the characters of the families above indicated.
TurrENTERATA - Family 1. Lingulidx. - Shells generally either oblong or circular, with a peduncle, sometimes of considerable length, passing out between the valves or through a narrow channel in the hinge margin ; texture horny ; no calcified supports for the labial appendages ; the fleshy spiral coils directed upwards. This family would comprise the following genera: Limmgula, Lingulella, Liugulops, Lingulepis, Glottidia, Honobolina, Obolus, Obolella, Digno7nia, Schmidtia, Acritis, Tolborthia, &c. Lingulella is one of the oldest known types of animal life, while Lingtda appeared for the first time about the middle of the Cambrian period, and has continued to be represented up to the present time.
Family 2. Discinidce. - Shells more or less circular or oval shaped, attached by a peduncle passing through a foramen in the ventral valve ; shell calcareous or horny ; seta; extremely long, barbed with cilia of great length ; labial appendages fleshy, curved backwards, with small terminal spire directed downwards as in Crania (fig. 11). Genera - .Discina, Trematis, Discinisca, Kutorgina (1), Acrotrata (1), Siphonotrata (1). Discina appeared about the middle of the Cambrian period, and has continued to exist up to the present time.
Family 3. Craniadoe. - Shells orbicular or limpet-like, entirely free or attached by a greater or lesser extent of the under surface of their ventral valve ; labial appendages spirally coiled, directed towards the bottom of the dorsal valve (fig. 11); shell calcareous, perforated by minute canals. Genera - Crania, Craniops, araM.SCUS, Pliolidops. The genus Crania appeared for the first time during the Silurian period, and has continued to be represented up to the present time.
Family 4. Trimerellidx. - Shells transversely or longitudinally oval; ventral valves usually the largest and flattest, with a more or less developed beak and area; ventral valves generally the most convex ; hinge rudely or faintly dentary ; all the genera, are provided with a solid or vaulted muscular platform in the interior of both valves ; no calcareous support for the labial appendages ; shell calcareous, and in two of the genera very massive. All the forms are extinct. Genera - Trimnerella, Mono2nerella, Dinobolus. The species of this family are restricted to the Silurian period.