26 ZOOLOGY SECT. 



In the Salpse and in Doliolum and Octacnemus the alimentary 

 canal forms a relatively small dark mass the so-called nucleus 

 towards the posterior end of the body ; it consists of oeso- 

 phagus, stomach, and intestine, the anal aperture being situated 

 in the peribranchial or atrial part of the internal cavity. 



The heart in all has the simple structure already described in 

 the simple Ascidian. In Appendicularia its wall consists of only 

 two cells. In Oikopleura it is apparently absent. 



The nervous system in Appendicularia consists of a cerebral 

 ganglion (Fig. 680, ne. gn.) at the side of the mouth on the dorsal 

 side, of a dorsal nerve which passes from this to a caudal ganglion 

 (ne. gn'.) at the root of the tail, and of a caudal nerve (ne'.) which 

 passes from this to the extremity of the tail, presenting at intervals 

 slight enlargements from which nerves are given off. An otocyst 

 (oto.) and a pigment-spot are placed in close relation to the cerebral 

 ganglion, and close to it also is a tubular process opening into the 

 branchial sac and evidently representing the duct of the sub- 

 neural gland of the simple Ascidian. In the simple Ascidians, 

 as we have seen, there is a single flattened ganglion, representing 

 the cerebral ganglion of Appendicularia, situated between the 

 oral and atrial apertures ; and the same holds good of the com- 

 posite forms. Many of the simple Ascidians have pigment-spots, 

 probably of a sensory character, around the oral and atrial aper- 

 tures. In Salpa and Doliolum there is also a single ganglion 

 (Figs. 683, 684 and 685, ne. gn.) situated dorsally, giving off nerves 

 to the various parts of the body. In Salpa there is an eye of a 

 simple character and an otocyst placed in close relation to the 

 ganglion in addition to eye-like bodies devoid of pigment : in 

 Doliolum these are absent, but pigment spots occur in the lobes 

 surrounding the oral opening. A subneural gland and duct are 

 present in both these genera. 



In the simple Ascidian we have seen that the renal organ 

 consists of a number of large clear vesicles situated in the loop of 

 the intestine and devoid of duct. In some forms the terminal 

 portion of the spermiduct has glandular walls in which concretions 

 of uric acid have been found. The sub-neural gland is by some 

 zoologists looked upon as perhaps having an excretory function. 



Reproductive system. The Urochorda are hermaphrodite. 

 Ovary and testis are in all cases simple organs placed in close 

 relation with one another. In Appendicularia (Fig. 680) they are 

 situated in the aboral region of the body. In the simple Ascidians, 

 they may be either single or double, and their ducts, sometimes 

 very short, sometimes more elongated, open close together into the 

 atrial cavity. In Pyrosoma there are no gonoducts, the ovary, 

 which contains only a single ovum, and the testis being lodged in 

 a diverticulum of the peribranchial cavity. In Salpa also the 

 ovary contains usually only a single ovum : ovary and testis lie in 



