pair organs pairs somite exopodite endopodite
THE APPENDAGES. - Just as we find a typical number of twenty-one body segments to prevail among the Crustacca, so also iu the appendages the type number of joints is seven, any departure from which is disguised by fusion of one or more joints together, the obsolete condition of others, or the depauperization of the limb into numerous artic2di (see fig. 5). At the coxal joint, or protopodite, each limb usually bifurcates ; firstly, there arises the external limb proper, or exopodite, which is normally seven-jointed ; and secondly, the internal branch, or endopodite, which may serve as a mouth organ, a bronchial appendix, a swimming organ, or a protection for the ova or the young. The appendages belonging to the three divisions of the body differ from each other in a greater or less degree in proportion to the higher or lower grade which we examine. Thus among the Decapoda (crabs and lobsters) the cephalic, thoracic, and abdominal somites all possess appendages with well marked characters, each series being highly differentiated for the functions to be performed by it. Among the Entomostraca, the appendages of the anterior cephalic somites alone are highly specialized, the others being either mere vegetative repetitions of one another, or else altogether wanting.
C = eephalon; 13, Th = thorax, showing the apodemata; Ab = abdomen. 1, Eyes; 2, antennnles; 3, antenna; 4, mandibles; 3, first maxilla; 6, second maxilla; 7, first maxillipedes; 8, second maxillipedes; 9, third pair maxillipedes. 10, One of the antepenultimate pair of thoracic legs of female; p. protopodite ; ep, epipodite; 9, gill. 11, One of the last pair of thoracic limbs in male ; p, protopodite. 14, Third abdominal somite ; ex, exopodite; en, endopodite, 7= labrum; rn = metastoma. 15-20, abdominal segments ; T, 21st segment, or teison.
which are never diverted from their normal use, though sometimes atrophied. In the more highly cephalized forms, the Decapoda13rachyura (erabs), the eyes are placed on the outer side of the two pairs of antenna, but the anatomical evidence shows that the most anterior pair of nerves, in this as in all other orders of Crustacea, is that which is connected with the ophthalmic organs.
Second pair. - The second somite bears the first pair of antenna (or antennules), called the inner pair in the higher forms, and the upper pair among the lower Crustacea. Ordinarily they are slender, tapering, freely-moving, multi-articulate organs. In the lobster both the endopodite and exopodite are equally developed from a common basal-joint or protopodite (see. 2 in fig. 6).
Third pain - The third. somite supports the second or posterior pair of antenna', sometimes called the outer pair in the higher, and the inferior pair in the lower forms of Crustacea. In the lobster these are represented by a basal-joint or protopodite, and a long filamentary, multi-segmented. endopodite, to the outer base of which a small scale is attached. representing the exopodite. In all the higher Crustacea the three most anterior somites are always pro-oral, being in front of the buccal orifice, and in most genera, moreover, they are specially set apart as bearing the organs of sense. Generally they are so closely blended together, both in the earlier stages of development and also in the adult forms, as to defy separation, the presence of three somites being only demonstrable by dissection, and by a knowledge of the fact that each pair of appendages, wherever it exists, presupposes a segment or ring to which it belongs. The genus Squilla affords, perhaps, the best evidence of the separate existence of these cephalic rings. In it the first, or ophthalmic somite, is quite distinct from the second and third antennary, which are also separable from one another, although the latter blends with the next somite, and the succeeding ones can only be distinguished by dissection. Usually the antenna or feelers are constant appendages ; but still, the number and disposition of these organs varies extremely. Some of the lowest forms are wholly without antenna, or are furnished with them in a merely rudimentary state. Some species have only a single pair, the normal number, however, as we have already pointed out, is two pairs. In position they are inserted on the superior or inferior surface of the head, according to the development of the somites composing the eephalon. Ordinarily they are slender flexible multi-articulate appendages, but even among the higher forms they are subject to extraordinary modifications ; thus in the Scyllarida, the external pair are developed into broad flat organs of natation, and probably also for burrowing. In Arcturus, an Isopod, they are the nursery for the young ; in the Entomostraca they are usually natatory organs ; in Pterygotus and Limit/us they are chelate, serving, as in the Copepoda, as clasping organs for the male. In the last free condition of the Cirripedia and Plizocephala they serve as the organs for attachment, - being converted into cement-duets in the former, and into root-like organs of nutrition in the latter. The second pair of antenna and sometimes even the first pair become mouth organs in the Merostomata.
Fourth to Ninth pairs. Epistomial or Mouth Organs. - In the Decapoda the six succeeding pairs may be called mouth-organs or epistomial appendages, being all engaged in duties subservient to nutrition. The fourth somite bears the actual jaws or mandibles proper with their palpi, outside which lie two pairs of maxilla, followed by three pairs of maxillipeds or jaw-feet. In each successive somite, these organs become less highly specialized mouth-organs, and betray the fact that they are after all only simple feet modified. Thus in Squilla (a Stomapod) the eighth somite (first thoracic) bears a pair of robust claws, the terminal joint of which is furnished with long and sharp teeth, these forming the principal organs of prehension ; whilst the ninth somite bears a pair of ordinary feet like the two following pairs (see fig. 71).
In most of the Edrioplithalinia the mouth-organs extend only to the seventh somite, the eighth and ninth being included with the ambulatory members.
In the Decapoda we can detect the more or less rudimentary endopodite and exopodite in the fifth pair of appendages, and in each succeeding pair to the ninth ; the eighth and ninth pairs also bear a third organ called an epfpodite, and a gill or branchial organ.
Tenth to Fourteenth Pairs. - In the higher forms the five somites which follow (and. which might be termed the postoral somites) bear the true walking limbs (pereiopoda, Spence Bate), the first pair of which in the Decapoda are usually developed into powerful chela, and serve as the chief organs of prehension.
These podites are usually seven-jointed, and each bears a gill on its basal-joint. They are formed by the endopodite, the exopodite being present only in the larval-limb of the Decapoda ; but in the adult Mysis (Stomapoda), eight pairs of limbs (that is to say, the five pairs of pereiopodites or " walking-feet," and the three pairs of maxillipeds or " jaw-feet,") are all furnished with two branches, one the endopodite, the other the exopodite, as in the larval Decapod.
Fifteenth to Twentieth pairs.--The next six somites bear each a pair of swimming-feet (or pleopodites). In the Decapoda-Brachyura these remain (like the segments on which they are borne) as eatremely rudimentary organs furnished with hairs which serve for the protection of the eggs after extrusion. In the Macroura they assist in swimming, and are composed of a simple exopodite and endopodite. The sixth pair are developed into broad plates, forming the lateral lobes to the tail.
The Twenty-first or caudal segment is destitute of appendages,.. and has therefore been considered by Prof. Huxley and others as a "median appendage," and not as a somite.' It varies in form, being sometimes a broad flat plate in the Decapoda, or a minute terminal one in the Amphipoda, or greatly developed as a roof to the branehi in some lsopoda, or forming a long terminal spine in the Xiphosura.