30 ZOOLOGY SECT. 



consists of only two cells. In one of the genera of Larracea 

 (KowalevsMa) it is absent. 



The nervous system in Appendicularia consists of a cerebral 

 ganglion (Fig. 723, w. gn') on the dorsal side of the mouth, 

 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 

 extends from this to the extremity of the tail, presenting at 

 intervals slight enlargements from which nerves are given off. 

 An otocyst or statocyst (ofo.) and a pigment-spot are placed in close 

 relation to the cerebral ganglion, and close to it also is a ciliated 

 funnel ; but there is no neural gland opening into the branchial 

 sac. In the simple Ascidians, as we have seen, there is a single 

 flattened ganglion, representing the cerebral ganglion of Appen- 

 dicularia, situated between the oral and atrial apertures ; 

 and the same holds good of the composite forms. Many of the 

 simple Ascidians have pigment-spots, probably of a sensory 

 character, around the oral and atrial apertures. The dorsal 

 tubercle is always present, but varies in shape in accordance 

 with variations in the form of the ciliated funnel, which always 

 opens on it in conjunction with the duct of the neural gland, 

 of which it forms the terminal part. The opening may be 

 divided into several smaller apertures by fusion of its lips : 

 rarely the duct gives off branches with independent openings. 

 The tubercle with the ciliated funnel is supplied with nerve- 

 fibres from the ganglion, and is probably a sensory organ of 

 some kind. The neural gland is usually below the ganglion, but 

 may be situated above it or on one side. Sometimes it coalesces 

 with it. 



In Salpa and Doliolum there is also a single ganglion (Figs. 

 726, 727, 728, and 731) situated dorsally, giving off nerves 

 to the various parts of the body. Salpa has a single tentacle, the 

 so-called languet (Fig. 728, Ing.), absent in Doliolum. In Salpa 

 there is a median horse-shoe-shaped eye (Figs. 728, 731), with 

 sometimes smaller accessory eyes. In Doliolum an eye is not 

 developed, but there is a pair of otocysts or statocysts. A neural 

 gland and duct with ciliated funnel are present in Doliolum; 

 but in Salpa, though there are a pair of glands which occupy 

 a position similar to that occupied by the neural gland (Fig. 

 731, n. gl.), their correspondence with the latter is not established, 

 and their ducts have no connection with the ciliated funnel. 



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 sperm-duct has glandular walls in which concretions 

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

 zoologists looked upon as having an excretory function, but there 

 is no positive evidence in favour of this view, and no definite 



