206 THE VOYAGE OF H.M.S. CHALLENGER. 



Ascidia placenta, Herdman (PI. XXXI. figs. 1-3). 



A*ci<Uaplar<nta, Herdman, Prelim. Rep., Proe. Roy. Soc. Edin., 1879-80, p. 715. 



External Appearance. — The body is elongated, oblong, elliptical or oval in shape, and 

 is flattened laterally ; the anterior end is slightly the narrower, and the posterior end is 

 rounded. It is attached by a small area a little posterior to the middle of the left side. 

 The apertures are both on the right side ; they are sessde, and not conspicuous. The 

 branchial is nearly median and terminal ; the atrial is a short distance from the dorsal 

 edge, nearly half-way down from the anterior to the posterior end. 



The surface is rather creased by seams running for the most part longitudinally, and 

 is slightly roughish or velvety. 



The colour is a yellowish-grey or horn colour. 



Length of the body, 6 "5 cm.; breadth of the body, 4 cm. 



The Test is rather thin, soft, and easily torn. It is roughish about the base of attach- 

 ment. The inner surface is smooth and glistening. Vessels are feebly developed. 



The Mantle is moderately muscular. 



The Branchial Sac is delicate, and is slightly plicated longitudinally. The transverse 

 vessels are all of one size. The internal longitudinal bars are moderately strong, and 

 bear at the angles of the meshes very long curved papillae, connected by delicate transverse 

 membranes. The stigmata are long and narrow, and are usually wider than the inter- 

 stiomatic vessels ; there are eight to twelve in a mesh. The meshes are large and rather 

 longer transversely than antero-posteriorly. 



The Dorsal Lamina is not ribbed, or only slightly so in places. Every here and there 

 a large tooth in the form of a short finger-like process is present, and between these are 

 usually three or four smaller ones. 



The Tentacles are filiform, about twenty-four in number, and all of one length. 



The Dorsal Tubercle is longish elliptical in shape, and is placed antero-posteriorly, with 

 the opening at the anterior end. 



This species is much compressed laterally, and as its point of attachment is near the 

 centre of the left side, it must have lived in a more or less horizontal position, as an 

 expanded flattened cake-like structure (PI. XXXI. fig. 1). In external appearance it 

 resembles Ascidia tenera somewhat, but they differ greatly in detafls of structure. 



The branchial sac is very delicate (PL XXXI. fig. 2), and the stigmata are long and in 

 some places rather irregular. The papillae are very large, and are found bent in all direc- 

 tions. The smaller intermediate ones which are occasionally present may be connected 

 by delicate transverse vessels, much slighter than the usual ones. 



The dorsal lamina is usually devoid of transverse ribs, as represented in figure 3, but 

 in some parts of its extent very slight ribs are present, running towards the larger 

 marginal teeth. 



