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



The genus was formed for the reception of a single species from deep water, 

 A byssascidia ivy villii. 



Abyssascidiawy villii, Herdman (PL XXVII. ). 



Abyssaseidia wyvillii, Herdman, Prelim. Rep., Proc. Roy. Soc. Edin., 1879-80, p. 470. 



External Appearance. — The body is irregularly oblong in shape, rather pointed at the 

 anterior, and rounded at the posterior, end. It is attached to a small manganese 

 nodule by the lower (ventral) surface in front of the middle. The lateral edges are rudely 

 parallel. The body is flattened dorso-ventrally, so that the branchial aperture being 

 anterior, the atrial is on the upper surface, three-quarters of the way to the posterior 

 end, and rather to the right of the middle. In consequence of this, more of the left than 

 of the right side enters into the formation of the upper surface. The branchial aperture 

 is at the edge, sbghtly to the right of the anterior end, and has twelve or fourteen 

 lobes ; the atrial has eight or nine lobes, both apertures are sessile. 



The surface is smooth. The colour is a very light grey, almost transparent. 



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



TJie Test is thick ; it is rather solid, but not hard, and is transparent. No vessels are 

 visible. The test contains only small fusiform cells in the hyabne matrix ; there are no 

 bladder cells. 



Tlie Mantle is very thin, the endostyle and viscera being seen distinctly through it. 

 A few large distant muscle bands run round the right edge, and extend over the left 

 side nearly as far as the endostyle. The atrial siphon is prominent, and provided with 

 fine muscle bands ; the branchial is also muscular, but not projecting. 



Tlie Branchial Sac is large, and fills the whole mantle cavity ; it is not" plicated, and 

 its structure is simple. Every alternate transverse vessel is slightly wider than the 

 intermediate ones, and here and there the stigmata extend from one larger vessel to the 

 next, cutting through the intermediate smaller one. The internal longitudinal bars widen 

 slightly at each intersection with a transverse vessel, but bear no papillae. Tusk-shaped 

 ducts, to which horizontal membranes are attached, connect the transverse vessels with 

 the swellings on the internal longitudinal bars. The stigmata are rather wide and 



o o o 



irregular ; there are about three in a mesh, which is generally nearly square. 



The Dorsal Lamina is reduced to a series of conical languets, which are blunt, com- 

 paratively short, and irregular in size. 



Tlie Tentacles are few, distant, small, and filiform ; there are two at each side of the 

 anterior end of the endostyle, and a few others in the usual circle, but separated by nearly 

 their own length from each other. 



The Dorsal Tubercle is carrot-shaped, tapers posteriorly, and has no visible aperture. 



Tlie Nerve Ganglion is elongated. It is placed at a considerable distance from the 

 dorsal tubercle. 



