SOME ASC1DIANS FROM THE ISLE OP WIGHT. 133 



what larger individual (fig. 1) the intermediate or quaternary vessels 

 are much more highly developed, and there is less difference between 

 them and the other horizontal vessels. There is no pharyngo-cloacal 

 slit.* 



The oesophagus opens into the pharynx high up on its dorsal edge, 

 half way between the cloacal siphon and the poterior end of the body. 

 In the largest specimen there are six primary horizontal bars between 

 the oesophageal opening and the posterior end of the pharynx. 



All my specimens are immature ; in even the largest individual the 

 development of the generative organs is still incomplete, and the ducts 

 are very slender in form ; while in smaller specimens the gonads are 

 quite rudimentary. 



In addition, however, to the specimens of which the above account 

 has been given, I took another Ascidian which there is every reason to 

 believe to be an adult individual of the same species, but which, from 

 its exceptional shape, is at least au abnormal one, so that I have 

 excluded it from the general description. 



It is represented of the natural size on PI. V, figs. 5a and 5b, and 

 deprived of the test, by fig. 6. The body is not compressed from side 

 to side (right to left morphologically) like an ordinary Ascidian and 

 like normal individuals of the same species, but dorso-ventrally ; 

 and thus it comes about that, although attached in the usual manner 

 by its left side, its right side does not present a flattened surface, but 

 is elevated so as to form a thickened longitudinal ridge of considerable 

 height. 



The dimensions are as follows : 



Length (antero posterior) ... 2 inches. 



Breadth (dorsal-ventral across the plane of 



attachment) . . . . | „ 



Thickness (morphological right to left) . | „ 



The breadth becomes considerably reduced towards the summit of 

 the ridge which represents the right side of the body. 



The test is very thick and presents all the characters of normal 

 individuals of Ascidia mollis, except that it is much corrugated on that 

 face of the body which contains the cloacal aperture (see fig. 5a). 

 The oral aperture is sub-terminal and on the same side as the cloacal 



* See page 144. 



