XIII 



PHYLUM CHORDATA 



21 



neural gland (Figs. 716, 718, %>. ; Fig. 720, gld., and Fig. 721, 

 n.gl.) — which has sometimes been correhited with the hyiK.'phyds 

 of the Craniata. A duct (Fig. 720, dd. and Fig. 721, (jl.d) runs 

 forward from it and opens into the cavity of the pharynx ; the 



eot ---^■.■■' ■(^'■' ■ 'j^}' 

 ^ ■..-:■ ■■ w " o.y 



lSiSi£SS^^ ■ L' i Peribranchial 

 cavity 



e.p br 



d.bl.B 



Branchial sao 



B 



d.D 



Fig. 721.— Antero-dors.al part of Ascidia, showing the relations of tlie hiyers of the body and 

 of the nervous system, A, in sagittal section ; B, in transvei'se section, d. 01. s. dorsal blood- 

 ■• sinus ; d. I. dorsal lamina ; d. a. dorsal nerve ; d. t. dorsal tubercle ; ert. ectoderm ; en. 

 endoderm ; c.p.br. epithelium of peribranchial cavity ; r/l. d. duct of neural gland ; I. v. points 

 to the ciliated epithelium covering a longitudinal vessel of branchial sac (pharynx) ; m. 

 mantle; n. nerve; n. ;/. ganglion ; a. <jl. neural gland; p. br. peribranchial cavity ; pp. h. 

 peripharyngeal bands ; rph. branchial siihiucter ; U, t', test ; tn. \ tentacle. (After Herd- 

 man.) 



termination of the duct is dilated to form the ciliated funnel, and 

 this is folded on itself in a complicated way to form a tubercle, the 

 dorsal tuhercle, which projects into the cavity of the pharynx. 



The excretory system is represented by a single mass of clear 

 vesicles, without a duct, lying in the second loop of the intestine. 

 In the interior of these are found concretions containing uric 

 acid. 



Reproductive System, — The sexes are united. The ovary 

 and the testis are closely united together, and lie on the left-hand 

 side of the body in the intestinal loop. Each of them contains 

 a cavity which, like the pericardium and the cavities of the 

 nephridial vesicles, forms a part of the original coelome. Con- 

 tinuous with the cavity of each is a duct — oviduct or sperm-duct, 

 as the case may be — which opens into the atrial cavity close to 

 the anus. 



The development of the Ascidian is described below (p. 32). 



2. Distinctive Characters and Classification. 



The Urochorda are Chordata in which the notochord is confined 

 to the tail region, and, in all but the Larvacea, is found only in 



