122 SOME ASCIDIANS FEOM THE ISLE OF WIGHT. 



connecting ducts themselves are sub-triangular in shape when seen 

 in profile. The horizontal and internal longitudinal vessels delimit 

 mesheSj which are sometimes almost twice as long as broad, and con- 

 tain four or five stigmata each. The stigmata are elongated, with 

 rounded ends ; they are frequently double, and then consist of an 

 anterior and a posterior portion of elliptical shape. The pharyngeal 

 wall is minutely plicated in a longitudinal direction. The meshes 

 almost invariably show some trace of a division into two equal 

 portions by the formation of an incomplete quaternary series of 

 horizontal vessels ; the extent to which this process is carried out 

 varies in different individuals and in different parts of the same 

 pharynx. The process is interesting, and may be completely traced 

 in fig. 4. A small projection arises from the internal face of an 

 interstigmatic bar, at its middle point (see fig. 4, upper row, 

 third mesh from the left), and is joined by a similar projection 

 from the opposite wall o£ the stigma (see the mesh below). The 

 concrescence of the two projections forms a horizontal bridge 

 across the middle of the stigma. The formation of several such 

 bridges across adjacent stigmata thus gives rise to a small horizontal 

 vessel (see the mesh below), which may be said to form part of a 

 quaternary series ; these quaternary vessels {h. v. 4) may even form 

 connections with the internal longitudinal bars beneath the inter- 

 mediate papillae {i. p.) of those structures. My figure represents 

 the condition of the branchial apparatus in the individual shown in 

 fig. 3 ; but in a somewhat larger individual (fig. 1) the inter- 

 mediate 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, 

 halfway between the cloacal siphon and the posterior 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 j 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 an abnormal one, 

 so that I have excluded it from the general description, 



* See p. 132. 



