body balanoglossus collar
THE HEMICHORDA. comprise the single genus Balanoglossus - formerly classified by Gegenbaur as Enteropneusta, an independent phylum of the animal kingdom. They are Vertebrata of worm-like form, the opercular epipleural folds of Cephalochorda and Craniata. This agreement is supported by the existence of a pair of collar pores opening into the ecelom of the collar, as the " brown funnels" of .Amphioxus open into the epipleural &elm of that animal. A proboscis pore, opening on the left side into the pneoral ccelom of the proboscis (paired in B. Icuptferi), is exactly representative of the similarly placed pore which in the young Amphioxas (according to Hatschek, 15) leads into the tubular organ derived from the left eoelomic chamber of the prmoral lobe of that animal. The whole surface of the body is ciliated, as in Nemertiues and Echinoderms, and as in no other Vertebrates. Following the collar is a perforated region of the body, - gill-slits opening from the outer surface into the pharynx. In the young form there is for a time, as in 21ppendicularix and the Ascidian tadpole, only one pair of gill-slits, but they subsequently increase in number as the animal grows in length. They resemble in form and structure those of Amphioxus. The notochord (h in fig. 10) arises at the anterior end of the hypoblast in the young, and grows forward, forming a support for the base of the proboscis. It is limited to this very small region. The cerebro-spinal nerve-cord originates by a delamination of a solid cord of epiblast in the mid-dorsal line of the middle third of the body ; then by invagination of its two ends it extends as a tube both anteriorly and posteriorly. A general network of nerve-fibres (and cells ?) exists beneath the epidermis all over the body. The blood-system is peculiar, consisting of an anterior heart and a dorsal and ventral vessel ; these are united by a plexus of subcutaneous vessels. The musculature of the body-wall is not broken into successive myomeres • but, on the other hand, the gonads (ovaries or testes) are sac-like, and, as in Amphiaxus, are repeated in a series throughout a great length of the body. In the pharyngeal region the gonad sacs agree in number with the gill-slits. There are no nephridia (unless proboscis pore and collar pores are to be so regarded); but the connective-tissue cells of the body-cavity are active as excreting agents, as in Echinoderms and in Urochorda, and a large glandular organ in the proboscis attached to the end of the notochord appears to have to do with this function. Not the least remarkable fact about Ilemichorda is the nature of their larvae. No other Vertebrata present larval forms which indicate the nature of the early ancestral history in what we may call pm-chordal times ; however interesting the Ascidian larva, or the young Ampitioxus, and the embryo dog-fish, they do not take us out of the Vertebrate area. Some species of Balafloglossits (? B. minutus), however, pass through a banded ciliate larval condition, which was known as Tornaria, and was considered to be an Echinoderm larva allied to Bipinnaria, before its relation to Balanoglossus was discovered. It is not possible to view the Tornaria larva of Balanoglossus as otherwise than identical with Echinoderm larva, and it results that Balanoglossus and the Echinoderms have remote genetic affinities of a special kind.
No classification of Homichorda is possible beyond an enumeration of the species :- Balanoglossus clavigerus (Della Chiaje), Naples.
B. minutus (Kowalewsky), B. kowalewskii (Al. Agassiz), east coast, enited Statue.
B. braoksii (Bateson), B. salmoneus (Giard), Brittany.
I'. robinii (Gland), I I It seems that in Balanoglossus we at last find a form which, though no doubt specialized for its burrowing sand - life, and possibly to some extent degenerate, yet has not to any large extent fallen from all ancestral emirs- once. The ciliated epidermis, the long Worm-like form, and the complete absence of segmentation of the body-muscles lead us to forms like the Nemertines. The great pro- c boseis of Balanoglossus may well be compared to the "1',m invaginable organ similarly placed in the Nemertines.
The collar is the first commencement of a eras structure destined to as-