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



not far distant. It is an elongated, fusiform, thin-walled sac, and is enclosed by a 

 membranous pericardium composed of a thin layer of connective tissue uniting an open 

 network of branched spicules similar to those found in the branchial sac, endostyle, &c. 

 This is lined by a layer of squamous epithelium. The cells are large, polygonal in 

 outline, and distinctly nucleated (PI. IX. fig. 11). 



The wall of the heart itself is composed of regularly arranged fusiform cells, inter- 

 locking with each other (PI. IX. fig. 9). These cells are about twelve to sixteen times 

 as long as they are broad, and are perfectly symmetrical, tapering gradually towards each 

 end. They have large central distinct nuclei, which stain brilliantly in carmine and 

 logwood. These cells are all distinctly striated transversely (PI. IX. fig. 10). The 

 striae appear as dark bands, leaving clear spaces between them ; there are usually about 

 t \vi nty in each cell. 



The Alimentary Canal. — The first portion of the alimentary canal, consisting of the 

 branchial sac or modified pharynx, has already been described ; there remain still to be 

 noticed the oesophagus, the stomach, the intestine, and the anus. 



The oesophageal opening lies at the posterior end of the dorsal lamina near the 

 posterior extremity of the branchial sac. It is an irregularly triangular aperture (PI. 

 VIII. fig. 8, ce.a.), surrounded by a membranous lip of considerable breadth, which is 

 the termination of the dorsal lamina. The oesophagus (ce.) is a short tube running in 

 a curved course posteriorly and ventrally so as to reach round the posterior end of th 

 branchial sac. It then opens into the irregular stomach (st.). This organ lies along the 

 ventral edge of the posterior end of the branchial sac, and has a remarkable appear- 

 ance (PI. VIII. fig. 8, St.). It is an irregularly elongated pyriform sac, having its 

 intestinal or anterior end rather the larger ; its walls are curiously folded and saccu- 

 lated, so that in external view it is multilobed, and the lobes are covered with bunches 

 of little rounded processes. From its anterior larger end the intestine emerges. It is 

 long and simple, but rather twisted in its course, performing several slight undulations. 

 It runs forwards along the ventral edge of the branchial sac for about half its extent, and 

 then, turning abruptly, it runs back almost parallel to the first part of its course, but lying 

 to the left of it, and after curving round dorsally it passes to the left of the stomach and 

 last part of the oesophagus, to terminate close to the atrial aperture (PI. VIII. fig. 8, a.). 

 Just at the most anterior part of the intestine where it turns round, an irregular process 

 projects from it into the narrow loop (PI. VIII. fig. 8, i.e.). This is apparently a caecum 

 — possibly it may be an individual abnormality. 



The whole of the intestine is thin walled, and its surface is smooth and of a dull 

 leaden-grey colour. The wall of the stomach is thicker, and is of a pale yellowish-brown 

 colour. There is no typhlosole or projecting fold of any sort in the intestine. The anal 

 termination is slightly widened, and has an undulating free edge (PI. VIII. fig. 8, a.). 

 The intestine contained fine mud of a dull brown colour. 



e 



