XIII 



PHYLUM CHORDATA 



17 



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

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

 which run 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- 

 msceral (card, vise.), breaks up into branches which ramify over the 

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

 visceral vessels or lacunae opens into a large sinus, the mscero- 

 Iranchial 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 direction 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 trans- 

 verse vessels, after undergoing oxy- 

 genation 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 con- 

 tractions take the opposite direction, 

 the course of this main current of the 

 blood is reversed. The cavity of the 

 heart and vessels is derived from the 

 blastocoele or primary body-cavity of 

 the embryo. 



The nervous system is of an ex- 

 tremely simple character. There is a 

 single nerve-ganglion (Figs. 374 and 

 376, ne. gn., and 378 gn.) which lies 

 between the oral and atrial apertures, 

 embedded in the mantle. This is 

 elongated in the dorso-ventral direc- 

 tion, 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 gland the sub- 

 neural gland (Figs. 674, 676, hyp. ; Fig. 678, gld.) which there is 

 evidence for correlating with the hypophysis of the Craniata. A 



VOL. II C 



FIG. 678. Ascidia. Hypophysis, 

 nerve-ganglion and associated 

 parts as seen from below, dct. duct 

 of hypophysis ; dors. lam. dorsal 

 lamina ; gld. subneural gland ; 

 gn. ganglion ; hyp. hypophysis ; 

 nv. nv. nerves ; periph. peri- 

 pharyngeal band. (After Julin.) 



