134 WYVILLE THOMSON, ON SYNAPTA INII^RENS. 



studded here and there with oval endoplasts, and continued 

 into various mesenteric straps and membranes. 



The mouth is a simple round orifice, surrounded, in a state 

 of contraction, by the epidermis thrown into folds by the 

 action of the sphincter muscles. The oesophagus is short and 

 highly muscular, and a second constriction separates it from 

 the alimentary canal. There is scarcely any distinction be- 

 tween stomach and intestine; the tube is nearly uniform 

 throughout. Its walls consist of three well-marked layers — 

 an external structureless epithelium, a portion of the general 

 epithelial lining of the body cavity ; a muscular layer, con- 

 sisting of a uniform bed of transverse fibres and of four deli- 

 cate longitudinal bands ; and of an inner layer, of variable 

 thickness, and finely granular. 



The alimentary canal passes nearly straight from the 

 oesophagus to the terminal anus; it is, however, somewhat 

 longer than the body in its ordinary state of rest, and it is 

 kept in position, and its slight convolutions are supported, by 

 a delicate mesentery of transparent fibres, with endoplasts 

 projecting from their surface, and by scattered transparent 

 cords of similar structure, which pass from the mesentery to 

 the walls of the body ca^'ity, with whose structm'eless inner 

 lining they seem to be continuous. Twelve transversely ob- 

 long plates form a regular calcareous dodecagon round the 

 oesophagus within the body-wall, and to these the fibrous and 

 muscular layers of the integument are attached. Five of the 

 plates are perforated. I have been unable to detect any 

 nerve-filament passing through the perforations, though 

 Miiller has observed that such is the case in some of the large 

 exotic species. Five longitudinal muscular bands, strongly 

 marked and shining white through the skin, and projecting 

 as raised straps about 3 mm. in Avidth and 1 mm. in thickness 

 into the body cavity, stretch from the five perforated plates 

 of the oesophageal ring, into which they are inserted, along 

 the whole length of the body, and are lost in the fibres of the 

 anal sphincter. The centre of the ambulacral system consists 

 of an annular canal, gii'ding the oesophagus immediately 

 within the calcareous ring. Twelve csecal processes from the 

 upper surface of this canal form the special lining membranes 

 of the twelve tentacles, and from its lower surface a long, 

 flask-shaped Polian vesicle on one side, and nearly opposite it 

 a single madreporic tubercle, capping a short, twisted canal, 

 liang freely into the cavity of the body ; and a delicate straight 

 vessel follows the course of each of the five longitudinal 

 muscular bands. 



