VERTEBRATA 



181 



Relation- ancestry. Its mode of life (burrowing in the sand in shallow water, 

 ship of whilst its general build is that of a swimming animal) and the 

 Cephalo- nature of its food (diatoms, &o., carried into the pharynx by ciliary 

 chotda to currents) in themselves suggest such a history. The vascular 

 Craniata system is elaborate in plan yet incomplete in detail, suggesting an 

 and the atrophy of its finer branches, which is consistent mth the small 

 Verte- size of AmpMoanis and the general principle that a complex vascular 

 brate system can only be developed in an animal which has attained to 

 ancestry, a certain bulk. The absence of well-developed sense organs and of 

 " cephalization " in an animal which has attained to such elabora- 

 tion of structure as is shown by the pharynx and atrial chamber, 

 and which has such well-developed njuscles to the body-wall, is an 

 inconsistency best explicable by degeneration ; so, too, the existence 

 of the elaborate series of fin rays, which are out of proportion to 

 the mechanical requirements of so small a form. 



Degenerate though AmpMoanis must be, the ancestor from 

 which it started on its retrogressive course was probably a long 

 way behind any living Craniate. There is no reason to suppose 

 that this ancestor had a cranium, or that the muscular segments 

 and segmental nerves in its cephalic region were fused and 

 welded. Amphioxus has probably lost, as compared with that 

 ancestor, lateral eyes and otocysts, nephridia, and, above all, size. 

 The epipleural folds which now form oral hood, branchial opercula, 

 and coalesced ventral fin were probably originally less developed 

 lateral ridges, protecting the gill-slits anteriorly and posteriorly, 

 serving by their undulations to assist in locomotion, whilst the 

 median fin and its rays were large and functional. 



One of the most curious features in the structure of Amphioxus 

 is its asymmetry. The anus is on the animal's left side ; the nasal 

 pit upon its left ; the myomeres on the two sides of the noto- 

 chord do not coincide ; and the right and left dorsal spinal nerves 

 do not arise ms-d,-vis to one another. There is no conclusive reason 

 for regarding this as an ancestral feature, although the early larval 

 form is as curiously asymmetrical as the adult. Amphioxus habitu- 

 ally rests upon the sand, lying upon one side of the body, and it is 

 possible that the distortion is related to this habit, as in the case 

 of the Pleuroneotid Fishes. 



However we may estimate Amphioxus, we are not led by it, though 

 its muscular metamerism is so well marked, a single step in the 

 direction of the Annelids, neither are we led directly, it is true, in 

 the direction of Nemertina in connexion with those points, as to 

 relationship of notochord with proboscis sheath and nerve-cord 

 with median dorsal nerve, insisted on by Hubrecht. But it will be 

 seen below that, by the agreement of Amphiooeus with Balanoglossus 

 in the structure of the perforations of the pharynx, in the possession 

 of collar pores, and in the praeoral glandular body, we do arrive at 

 an important connexion with Nemertine-like forms. 



The Ukoohorba. 

 Char- Urochorda, are Vertebrata which, with the exception of the group 



acters Larvalia (Appendimlaria, Fritillaria, Oikoplewra), have receded 

 of TTro- very far indeed from the characteristic Vertebrate structure, show- 

 charda, ing neither notochord nor nerve-cord, and gill-slits only of the 

 most highly modified and aberrant form ; some, however (certain 

 Ascidians), pass through a larval condition in which these struc- 

 tures are present in the normal form. It is necessary for the pur- 

 poses of the present article to confine our attention to Larvalia and 

 to the larval forms which retain ancestral characters. (For a de- 

 scription of the whole group, see the article Titnicata. ) In TJro- 

 ehorda thus signalized the notochord never reaches forward into the 

 anterior part of the body, but is confined to the tail (hence Uro- 

 chorda). The longitudinal muscles of the region traversed by the 

 notochord show traces of metameric segmentation, which are prob- 

 ably survivals of a more complete development of myomeres in 

 ancestral forms {i6). There is no trace in Larvalia of fin rays 

 or other skeletal structure. Corresponding to the opercular folds 

 and epipleura of Craniata and Cephalochorda are ridges of the 

 body-wall, which protect the pharyngeal gill-slits, and may give 

 rise, as in Cephalochorda, to an enclosed atrial chamber with 

 atriopore. The gill-slits in these larval forms are few in number 

 (one or two pairs), but in many of the aberrant Urochorda (by far 

 the majority of the group) they become excessively numerous and 

 complicated in structure, and are supported by a chitinous (?) 

 framework, as in Cephalochorda. It has been suggested that the 

 fenestrated structure of the pharyngeal wall in Tunioata does not 

 represent a series of gill-slits, but a single pair of slits subdivided. 

 This suggestion is worthy of further consideration. 



The cerebro-spinal nerve-cord is tubular and presents itself as a 

 dilated cerebral vesicle in front of the notochord, and as a narrower 

 part running along the whole length of the notochord. 



Sense organs are present— a single eye with pigment and lens, 

 a single otooyst, and an olfactory pit (Larvalia). The mouth is 

 dorsal in position in the Ascidian tadpole, but subtermmal in Lar- 

 valia. The pharynx is wide, and is followed by a narrow oeso- 

 phagus, stomach, and intestine, which does not open ventrally but 

 turns upwards to the anus. The Larvalia have a rudimentary 

 heart and no vascular system,— a fact connected with their diminu- 



tive size. For the same reason no vascular system develops in the 

 Ascidian tadpole until it has ceased to be locomotive and has entered 

 upon its later development ; but in the larger adult Urochorda a 

 contractile heart and a well-developed vascular system are present. 



No undfeniable nephridia are present in Larvalia nor in the larval 

 Ascidian, and no structure comparable to the collar pores of Balano- 

 glossus or the atrio-coelomic funnels of Amphioxus is known in them. 



The subneural gland, however, a glandular tube opening anteriorly 

 near the mouth of the pharynx, appears to be identicalwith the 

 praeoral larval gland oi Amphioxus and the proboscis pore and gland 

 of Balanoglossus. It is probably to be regarded as a nephridium, 

 and has been compared by Jiilin and Van Beneden to the pituitary 

 body of Craniata, with which it corresponds in position and de- 

 velopment. 



The gonads of Larvalia are developed in irregular masses on the 

 walls of the coelom, ovaries and testes in the same individual. 



As above indicated, there is a small section of Urochorda which Classi- 

 retain in adult life the tadpole-like form and the essential Verte- fioatioh 

 brate organs which are exhibited by the larvae only of other Uro- of Uro- 

 chorda, and by a few only of these. This necessitates a primary chorda. 

 division of the branch into two grades. 



Grade A. — Larvalia (Appendicularia, Fritillaria, Oikopleura). 



Grade B. — Saccata. 

 Class I. — Ascidix {Simplices, Sociales, Composite, Pyroso- 



Class II. — Salpiformia {Salpiidea, DolioUidea), 

 a 



Fia. ^.—^ntiXkma, (Appendiculariaifwrmta, one of the Urochorda. (Original 

 drawings.) A. Lateral surface view, showing habitual carriage of "hody" 

 at right angles to " tail." . B. Organs of body as seen by transparency. 0. 

 Lateral view of body with tail in morphological position, showing organs by 

 transparency. D. Surface view of animal from below to show apertures, a, 

 Otocyst in connexion with brain ; 6, olfactory pit ; c, dorsal hood ; d, nerve- 

 tube passing from enlarged brain to caudal region, where it forms one true 

 ganglion and a series of minor enlargements, corresponding to the rudimentary 

 " myotomes "or " myomeres " of the tail ; e, stomach ; /, ovary ; g, testis ; A, 

 notochord (urochord) ; i, nerve-tube or myelon in tail ; fc, fifth myomere of 

 tail ; I, anus ; m, heart ; n, gill-slit ; o, endostyle or hypobranchial groove ; 

 p, mouth. 



Urochorda are so extremely aberrant, and show so little more Relations 

 than a transient developmental indication of the essential Verte- of Uro- 

 brate organs, that we cannot hope to get much positive information chorda to 

 from them on the subject of Vertebrate ancestry. Only the minute Verte- 

 AppendicMlarise [Larvalia) retain the Vertebrate structure through brate 

 life, and they are obviously, on account of their minute size, ex- ancestry, 

 tremely degenerate. It is possible to make hypotheses as to the 



