ASCIDIA ALDERI. 99 



The mantle is rather delicate, with the muscular 

 fibres fine, and chiefly arranged transversely. 



The branchial sac (PI. XVII, fig. 7) extends a little 

 below the visceral mass ; it is minutely plicated ; the 

 primary vessels are pretty regular, and the stomata 

 of the secondary vessels are rather long, with rounded 

 extremities. The longitudinal bars are as numerous 

 as the plications, and the papillas are of moderate size, 

 with the points rounded and produced some way in 

 advance of the lateral tubercular projections, which 

 give to them a shouldered appearance ; thence the 

 papilla? widen to the base ; the small intermediate 

 ones are formed exactly like the larger papillae. The 

 papillary membrane is distinctly developed. The oral 

 lamina (PI. XIX, fig. 6) is wide, and is divided in front 

 much further down than usual ; it is ribbed .011 the 

 right side, and the margin is strongly pectinated, the 

 points being much elongated immediately in front of 

 the mouth, and increased in number, as here the pecti- 

 nations, which are usually the produced extremities of 

 the ribs, have others between them that originate in 

 the membrane of the lamina itself. The tentacular 

 points, of which there are about twenty-seven, on the 

 left of the mouth, are large, wide, triangular, leaf-like 

 processes. The branchial tubercle is small, and in the 

 form of a simple loop open in front. The tentacular 

 filaments (fig. 7) at the entrance of the branchial sac, 

 are arranged in a single line, not on the usual muscular 

 band, but apparently on a delicate fold of the lining- 

 membrane ; there are about twenty slender, not very 

 long filaments placed considerably apart, with 

 occasional short intermediate ones making in all 



o 



about fifty. 



The ovary is confined entirely to the left side of the 

 intestinal loop ; though the extremities of three or four 

 of the branches pass so far round the upper margin 

 of the intestine as to appear at the right side. The 

 organ is composed of numerous, irregularly-contorted 

 branches, some of which stray downwards as far as 



