Page:Encyclopædia Britannica, Ninth Edition, v. 24.djvu/207

Rh VERTEBRATA 187 greater or less elaboration of the ancestors of Urochorda, and to maintain even that their ancestry had reached as high a condition as that shown by Craniata ; on the other hand, it does not seem likely that their point of divergence from the main ancestral line leading to Craniata was lower than, or even so low as, that at which Amphioxus branched off. The differentiation of trunk and tail by the limitation of the notochord anteriorly is a nearer ap proach to Craniate structure than that shown by Amphioxus, whilst the definite development of a brain of considerable relative size places Urochorda nearer to the Craniates than is Amphioxus. The metameric myomeres so strongly developed in this last are not absent in Urochorda, as is often maintained, but exist in a rudimentary form, indicating that they had once a fuller develop ment. THE HEMICHORDA. !har- Hcmichorda comprise the single genus Balanoglossus formerly cters of classified by Gegenbaur as Enter opneusta, an independent phylum Jemi- of the animal kingdom. They are Vertcbrata of worm-like form, cc II Fir,. 10. Balanoglossus, anatomy and development. (Modified from Rateson.) A. Balanoglossus kowalewskii, Bateson; from the coast of Virginia, U.S.; natural size ; a, proboscis ; 6, collar ; c, perforate region ; d, flattened digest ive region; e, cylindrical hind region. B. Diagram of dorsal view, showing certain organs as though the body-wall were transparent. C. Diagram of a vertical antero-posterior section. D. Diagram of a dorsal view to show vessels and nerves by transparency. E. Diagram of a transverse section through the collar. F. Larva of B. kowalewskii ; diagram of horizontal sec tion. G. Vertical longitudinal section of an older larva of the same. Letter ing B-G : re, proboscis ; 6, collar ; /, nerve-tunic of proboscis ; g, proboscis pore (ciliated orilice) ; h, notochord (limited to a small tract of modified tissue derived from proeoral extension of alimentary canal); i, dorsal nerve-plate; k, collar-pore (right and left), opening to exterior from collar ccelom just be neath the collar ; I, continuation of dorsal nerve-plate as a nerve-cord ; m, pharyngeal perforations (gill-slits) ; n, coelom of proboscis (anterior azygos primitive coslomic pouch); n z , collar cojlom (right and left middle ca-lomic pouches of embryo) ; n 3 , body crelom (right and left posterior ccelomic pouches of embryo); o, mouth; p, ventral nerve-tunic of body- wall ; g, proboscis gland ; r, strands connecting dorsal nerve-plate with outer wall of collar ; s, cavity of pharynx in front of perforate region ; t, dilated part (heart) of dorsal vessel within proboscis-gland ; t, dorsal vessel ; u, blood- vessels of body- wall in section ; w, paired nerves of collar region in transverse section ; x, peri- hii-mal ccelom, surrounding dorsal vessel in collar region ; y, digestive region of gut (in embryo) ; 2, mesoblast. H. Larva of another species of Balanoglosstts, known as the Tornaria larva of Johann Miiller, and resembling an Echinoderm larva, aa, prseoral ciliated band of Tornaria ; 66, post-oral ditto ; cc, terminal ditto ; dd, mouth ; ee, apical plate and sense organ ; ff, canal system and pore ; gg, gut ; hh, anus. elongate and somewhat flattened from above downwards. In front of the mouth is a long cylindrical proboscis, and behind it a collar, the free margin of which is turned backwards, and corresponds to the opcrcular epipleural folds of CepJudochorda and Craniata. This agreement is supported by the existence of a pair of collar pores opening into the ccelom of the collar, as the &quot; brown funnels&quot; of Amphioxus open into the epipleural ccelom of that animal. A proboscis pore, opening on the left side into the pneoral ccelom of the proboscis (paired in B. kup/cri), is exactly representative of the similarly placed pore which in the young Amphioxus (according to Hatschek, 75) leads into the tubular organ derived from the left ccelomic chamber of the praeoral lobe of that animal. The whole surface of the body is ciliated, as in Nemertines 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 Ap- pcndiculariie 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 Amphi oxus. 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 sub cutaneous 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 Amphioxus, are repeated in a series throughout a great length of the body. In the pharyn- geal 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 Hcmichorda is the nature of their larvae. No other Vcrtebrata present larval forms which indicate the nature of the early ancestral history in what we may call prse-chordal times ; however interesting the Ascidian larva, or the young Amphioxus, and the embryo dog-fish, they do not take us out of the Vertebrate area. Some species of Balanoglossus (? B. minutiis), 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 Balano glossus as otherwise than identical with Echinoderm larvae, and it results that Balanoglossus and the Echinoderms have remote genetic affinities of a special kind. No classification of Homichorda is possible beyond an enumera- Species tion of the species : of Hemi- 1. Balanoglossus clavigerus (Delia Chiaje), Naples. cJlorda. 2. B. ininutus (Kowalewsky), ,, 3. B. kowalewskii (Al. Agassiz), east coast, United States. 4. B. brooksii (Bateson), ,, 5. B. salmonetis (Giard), Brittany. 6. P. robinii (Giard), ,, It seems that in Balanoglossus we at though no doubt specialized for its and possibly to some extent degenerate, extent fallen from an ancestral emin- dermis, the long Worm-like form, and segmentation of the body-muscles lead Nemertines. The great pro- may well be compared to the larly placed in the Nemertines. The collar is the first commencement of a structure destined to as sume great import ance in CepJMlo- chorda and Crani ata, and perhaps pro tective of a single gill slit in Balanoglossus bef the number of those tures had been extended rowing, as we may, the : from the Nemertines, and the lateral in addition to the dorsal nerve, i find that Balanoglossus gives the most hopeful hypothetical solution of the pedigree of Vertebrates. Space has not permitted us to go so fully into pros and cons as the speculative nature of the subject requires; but we give the final conclusion to which our consideration of the structure of the four great branches of the Vcrtebrata Ifads in the form of the accompanying genealogical tree. last find a form which, Relations burrowing sand - life, of Hcmi- yet has not to any large chorda ence. The ciliated epi- to Verte- the complete absence of brate us to forms like the ancestry, boscis of Balanoglossus invaginable organ simi-