28 



ZOOLOGY 



SECT. 



proc 



tent 



species, and has the oral and atrial apertures towards one end of the 

 body, which is somewhat discoid, with its margin produced into 

 eight tapering processes. It has no eye. In all probability 

 Octacnemus is more nearly related to the social Ascidians (p. 24) 

 than to Salpa. 



Pyrosoma (Fig. 738) is a colonial Tunicate, the colony assuming 

 the form of a cylinder, the internal cavity of which, closed at one 

 end, open at the other, serves as the common cloaca for all the 

 zooids. The oral apertures (Fig. 739, or. ap.) of the zooids are 

 situated on the outer surface of the cylinder on a series of papillae. 

 The colonies of Pyrosoma, which may be from jbwo or three inches 

 to four feet in length, are pelagic, and are brilliantly phos- 

 phorescent. 



The enteric canal in 

 Appendicularia (Fig. 732) 

 consists, in addition to the 

 pharynx, of a narrow oeso- 

 phagus, a bilobed stomach, 

 and a straight intestine 

 (int.) which opens directly 

 by an anal aperture (an.) 

 situated on the ventral 

 side. The alimentary canal 

 of the simple Ascidians has 

 already been described, 

 and there are few differ- 

 ences of consequence in 

 the various families, ex- 

 cept that in some cases 

 there is a well-developed 

 digestive gland or "liver"; 



FIG. 739. Part of a section through a Fyrosoma J/L nnirmndtp frvrms fhp 

 colony, air. ap. atrial aperture ; or. ap. oral aper- 3mpOSH 



ture ; proc. processes of test on outer surface of arrangement of the parts 



colony : ph. pharynx ; stol. stolon on which are de- . , . ,, . , 



veloped buds giving rise to new zooids ; tent, tentacles. IS tne Same 111 all essential 



respects as in the simple. 



In the Salpa3 and in Doliolum and Octacnemus the alimentary canal 

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

 wards the posterior end of the body ; it consists of oesophagus, 

 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 one of the genera of Larvacea (Kowalevskia) 

 it is absent. 



The nervous system in Appendicularia consists of a cerebral 

 ganglion (Fig. 732, ne. gri.) 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 



stoL 



