XIII 



PHYLUM CHORDATA 43 



istics of ihc Craniates. The complete want of segmentation 

 and the virtual absence of a crolome seem to point in the latter 

 direction : the presence in the larva of highly developed central 

 nervous system and sense-organs in the former. Ayipcndicularia 

 is hardly to be regarded as representing a primitive ancestral 

 type ; its close resemblance to the larva of the sessile 

 Ascidians rather seems to indicate that it is a persistent larval 

 fonn — a form in which sexual maturity has been reached at 

 earlier and earlier stages in the life- history, and in which the final 

 sessile stage has at last been lost, the animal having become 

 completely adapted to a pelagic life. Probably the other pelagic 

 forms — Salpa, Doliolum, Pyrosoma — were also descended from 

 sedentary ancestors : none of them show any character that can be 

 interpreted as primitive. 



The nearest existing ally of the Urochorda among lower forms 

 is probably Balanoglossus. The similarity in the character 

 of the pharynx, or anterior segment of the enteric canal, j)erforated 

 by branchial apertures, is alone sufficient to point to such a 

 connection ; and further evidence is afforded by the occurrence of 

 a "notochord" in both, and by the similarity in the development 

 of the central part of the nervous system. But the notochord of 

 the larval Ascidian, almost confined to a post-intestinal tail-region, 

 differs very widely from the structure in Balanoglossus suyoposed to 

 correspond to it, which is situated anteriorly and directed 

 forwards ; moreover, the other differences are so great that the 

 alliance cannot be a close one, and Balanoglossus and its allies 

 can only be looked upon as very remotely connected with the 

 stock from which the Urochorda are descended. Further considera- 

 tion will be given to this subject in the general treatment of the 

 relationships of the Chordata. 



SUB-PHYLUM III.— EUCHORDA. 



We have seen that the fundamental characters of the Chordata 

 are the presence of a notochord, of a dorsal, hollow, nervous system, 

 and of a pharynx perforated by apertures or gill-slits. In none of 

 the lower Chordata, however, are these structures found in a 

 typical condition, at least in the adult. In Balanoglossus, Cephalo- 

 discus and Rhabdopleura, the " notochord " is rudimentary, and in 

 nearly all Tunicata it is present only in the embryo. In 

 Rhabdopleura the gill-slits are absent, and in that genus as well 

 as in Cephalodiscus and the adult Tunicata the nervous system 

 is represented by a single solid nerve-centre or ganglion, the 

 neuroccele being absent. In Balanoglossus, moreover, there is a 

 ventral as well as a dorsal nerve-cord, and it is only in the anterior 

 portion of the latter that the neurocoele is represented. 



