20 



ZOOLOGY 



SECT. 



lining;', and of spaces or lacunip, forming a haoniocoele : in the 

 description that follows, therefore, the word vessel is not used in 

 its strict sense. At each end of the heart is given off a lai'gc 

 " vessel." That given off ventrally, the hranchio-cardiac vessel (br. car.) 

 runs along the middle of the ventral side of the phaiynx below 

 (externally to) the endostyle, and gives off a number of branches 

 which extend along the bars between the rows of stigmata, and give 

 off smaller branches passing between the stigmata of each row. 

 The vessel given off from the dorsal end of the heart — the cardio- 

 viseeral (card, vise.) — breaks up into branches wdiich ramify over the 

 surface of the alimentary canal and other organs. This system of 

 visceral vessels or lacunoe opens into a large sinus, the viscero- 

 hranchial vessel, which runs along the middle of the dorsal wall 

 of the pharynx externally to the dorsal lamina, and communicates 

 with the dorsal ends of the series of transverse branchial vessels 

 In addition to these principal vessels there are numerous lacunae 

 extending everywhere throughout the body, and a number of 



branches, given off both from the 

 branchio-cardiac and cardio- visceral 

 vessels, ramify, as already stated, in 

 the substance of the test. The direc- 

 tion of the circulation through the 

 main vessels differs according to the 

 direction of the heart's contractions. 

 When the heart contracts in a dorso- 

 ventral direction, the blood flows 

 through the branchio-cardiac trunk to 

 the ventral wall of the pharynx, and 

 through the transverse vessels, after 

 undergoing oxygenation in the finer 

 branches between the stigmata, reaches 

 the viscero-branchial vessel, by which 

 it is carried to the system of visceral 

 lacunae, and from these back to the 

 heart by the cardio-visceral vessel. 

 When the contractions take the op- 

 posite direction, the course of this 

 main current of the blood is reversed. 



The nervous system is of an ex- 

 tremely simple character. There is. 

 a single nerve-ganglion (Figs. 716 

 and 718, ne. gn., 720, ^?i., and 721, «.//.) 

 which lies between the oral and atrial 

 apertures, embedded in the mantle. This is elongated in the 

 dorso-ventral direction, and gives off at each end nerves which 

 pass to the various parts of the body. 



Lying on the ventral side of the nerve-ganglion is a bod}- — the 



Fifi. 720. — Ascidia. Ooi-sal tubercle, 

 nerve-ganglion, and associated 

 parts as seen from below, dct. duct 

 of neural gland ; dors. lam. dorsal 

 lamina ; gld. neural gland ; gn. 

 ganglion ; hyp. dorsal tubercle ; 

 ni:, nt\ nerves ; per!ph. peri- 

 pharyngeal band. (After Julin.) 



