42 ZOOLOGY SECT. 



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 supposed 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 consideration 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, Cephalodiscus, 

 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 neuroccele is represented. 



In the Euchorda, on the other hand, what have been called the 

 three fundamental chordate peculiarities are fully and clearly 

 developed. There is always a distinct notochord extending as a 

 longitudinal axis throughout the greater part of the elongated 

 body, and either persisting throughout life, or giving place to an 

 articulated vertebral column or backbone. The central nervous 

 system remains throughout life in the form of a dorsal nerve-tube 

 or neuron containing a longitudinal canal or neurocoele, and the 

 pharynx is always perforated, either throughout life or in the 

 embryonic condition, by paired branchial apertures or gill-slits. 

 In addition to these characters, the mouth is ventral and anterior, 

 the anus ventral and posterior ; the muscular layer of the body- 

 wall is primarily segmented, and the renal organs arise as a series 

 of paired tubules which may represent either nephridia or ccelomo- 

 ducts. Moreover, there is always an important digestive gland, 

 the liver, developed as a hollow outpushing of the gut, and distin- 

 guished by the fact that the blood from the alimentary canal 

 circulates through it before passing into the general current, thus 

 giving rise to what is called the hepatic portal system of blood-vessels. 



The sub-phylum Euchorda comprises two sections of very unequal 

 extent. 



