XI" PHYLUM CHOEDATA 21 



neural gland (Figs. 716, 718, %).; Fig. 720, ghl, and Fig. 721, 

 n.ql.) — which has sometimes been correlated with the hy'pfyphijsis 

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

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



Fio. 721. — Antero-dorsal part of Ascidia, showing the relations of the layers of the body and 

 of the nervous system, A, in saj^ittal section ; B, in transverse section, d. bt. s. dorsal blood- 

 sinus ; (I. I. dorsal lamina ; (/. n. dorsal nerve ; d. t. dorsal tubercle ; ect. ectoderm ; en. 

 endoderm ; c. y>. hi: epithelium of peribranehial cavity ; f/L d. duct of neural gland ; I. r. points 

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

 mantle ; 'it. nerve; n. ft. ganglion ; ii. f/l. neural gland ; p. hi: peribranehial cavity ; 'p'l'i' h. 

 peripharyngeal bands ; sph. branchial sphincter ; (., t . test ; tii. [ 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 tulerclc, which projects into the cavity of the pharynx. 



The excretory system is rejiresented 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 cadome. 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 Characteks 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 



