SOME ASCIDIANS PEOM THE ISLE OF WIGHT. 127 



mediate pectinations of its edge ; but the concave side, instead of 

 being smootli, as is usually the case in Ascidians, is profusely studded 

 with stout papilla, as shown in the figure. There is a certain ten- 

 dency of the papillas to be arranged in rows directed obliquely from 

 the summit to the free edge of the lamina, from before backwards ; 

 but this general tendency is frequently departed from. The dorsal 

 lamina is continued for some distance behind the oesophageal aperture. 



Branchial apparatus. — The horizontal vessels are often of two 

 sizes,* broad and narrow, and these vessels alternate with one another 

 in position ; but the breadth of the larger vessels never exceeds half 

 the antero-posterior diameter of the meshes — usually it is consider- 

 ably less. The pharyngeal wall is minutely plicated. The internal 

 longitudinal bars are slender in form. At their junctions with the 

 connecting ducts are situated blunt papillee of characteristic shape ; 

 they are of an erect discoid form, with a semi-circular edge, com- 

 pressed from before backwards, and provided with a supporting 

 ridge or buttress upon their anterior and posterior faces (fig. 11 6). 

 Usually the meshes are square, and intermediate papillee quite absent; 

 but in some parts of the pharynx transverse rows of meshes may fre- 

 quently be observed which are distinctly elongated in a longitudinal 

 direction, and in such regions minute intermediate papillae may be 

 detected upon the internal longitudinal bars. The elongation of 

 the meshes and appearance of intermediate papillas is preparatory 

 to the formation of a new series of horizontal vessels, in the manner 

 which I have described above in Ascidia mollis. There are four or five 

 stigmata in each mesh (fig. 11a). There is no pharyngo-cloacal slit. 



My largest specimen is mature, and minute white ova are present 

 in the oviduct. 



After much consideration I have arrived at the conclusion that 

 the specimens whose structure I have just described represent the 

 species Ascidia depressa of Alder, and that the Ascidians described 

 under this name by Heller, Herdman, and Koule, are distinct 

 from it. 



A reference to Alder's original account will show how perfectly 

 in every point my specimens agree with his description, with the 

 exception that I can make no statement as to the presence of red 

 ocelli around the apertures. I did not observe these spots in the 

 living animals ; but on the other hand I paid no attention to the 

 point, and probably overlooked their existence. In every other 

 respect the correspondence is complete, and I may draw especial 

 attention to the following details — the shape and colour ; the ex- 

 panded edge ; the position and form of the apertures ; the granula- 

 tions (minute tubercles) on the upper surface -, the absence of inter- 



* This distinction of size is much less apparent in mature than in young individuals. 



NEW SEKIKS. — VOL. II, KO. II. 11 



