GEROULD: CAUDINA. 147 
in sending nerves to the periphery of the body. Whether this 
couche profonde is homologous in the holothurians, echinoids, 
ophiurans, and star-fishes, can be decided only after more thorough 
investigations into its development in these various groups than 
have hitherto been made. 
6. DIGESTIVE SYSTEM. 
I shall first give an account of the general morphology of the 
digestive tract and then describe its histology. 
1. GENERAL MORPHOLOGY. 
The mouth, as in all the Molpadiidae as far as known, is cir- 
cular and occupies a central position at the anterior end of the 
body. The epithelium of this region is similar to that of the body- 
wall elsewhere, but the connective-tissue layer attains, when the 
animal is expanded, more than ordinary thickness. That part of 
the connective-tissue layer which lies immediately beneath the 
epithelium, and also that immediately in front of the buccal sphincter 
(Plate 4, fig. 44), is composed of closely packed fibers, parallel to 
the surfaces which they accompany; whereas loose fibers without 
definite direction fill the intermediate space. 
The sphincter of the mouth (Fig. 44, spht. buc.) is a modification 
of the circular pharyngeal muscle layer, as in holothurians generally; 
immediately behind the eonneetive tissue of the buceal region this 
layer spreads out like the mouth of a trumpet to form a thin flat 
ring. The most distal of the concentrically arranged fibers are only 
slightly removed from the axial margin of the nerve ring. 
Radial muscle fibers (Fig. 44, mu. v. buc.), which oppose the action 
of the circular ones and lie immediately behind them, extend from 
the wall of the pharynx to the axial wall of the tentacle, where they 
are attached. These fibers do not form a continuous layer, but are 
either isolated or in small groups. 
The pharynx extends from the mouth to the circular water canal, 
where, without any marked constriction, it joins the stomach. It 
gradually diminishes in caliber from before baekward, and is provided 
with the usual internal longitudinal ridges. The outer surface is 
