THE APODOUS HOLOTHURIANS 139 



fibers running to single neuro-epithelial cells. The bucco-pharyngeal nerves run 

 radially inward from the nerve ring to the buccal sphincter muscle and then 

 turn backward along the pharynx; they consist simply of nerve-fibers, among 

 which are interspersed ganglion cells ; they innervate the oral disc and the 

 pharynx, both muscles and epithelia. The arrangement of the bucco-pharyngeal 

 nerves in the interradii corresponds remarkably to the number of tentacles ; 

 there is in any given interradius one less bucco-pharyngeal nerve than the num- 

 ber of tentacles; thus the mid-dorsal interradius has three bucco-pharyngeal 

 nerves, while one of the ventral interradii has only one. Each radial nerve is 

 accompanied on its inner side by a tube-like cavity closed at both ends, called 

 the hyponeural canal. A similar space exists on the outer side of the radial, 

 on the upper and inner sides of the circumoral, and on the inner side of the ten- 

 tacle nerves, and these are known as the epineural canals. These canals are 

 lined, at least on the sides away from the nerves, by a very thin epithelium, and 

 are believed by Gerould ('96) (and Greeff, '72, and Herouard, '89) to be normal 

 structures, though other investigators believe them to be artefacts. 



BODY-WALL. The body-wall of the Molpadids is very variable in thickness, 

 according to the amount of connective tissue it contains. While frequently very 

 thin and translucent, it is in some species remarkably thick, and in old speci- 

 mens may measure several millimeters in a cross-section. It consists of five 

 parts, as in the Synaptidre : a cutis and a layer of epithelial cells which with the 

 cutis make up the epidermis ; a layer of connective tissue in which lie the cal- 

 careous bodies, pigment, etc. ; a layer of transverse muscles ; in the radii the 

 longitudinal muscles; and an inner epithelium, which lines the body-cavity. 



1. Epidermis. The cutis is a thin, transparent structureless layer secreted 

 by the epithelial cells and exhibiting no special peculiarities. The epithelial 

 cells are of the same three sorts which occur in synaptids, ordinary support- 

 ing cells, sensory cells, and gland cells. The former are the most abundant and 

 really make up the epidermis; they are vertically elongated, polygonal cells, 

 which taper into a point at the inner end, so that they extend into the connect- 

 ive tissue layer, and thus no sharp line of division between the latter and the 

 epidermis appears. On the digits of the tentacles these epidermal cells are as 

 much as 50 ^ in vertical length, but on the anterior part of the body they 

 are only 20 /*, and posteriorly still less. The sensory and gland cells are 

 scattered irregularly in the epidermis, but are much more abundant anteriorly 

 and especially on the tentacles. The former are usually isolated and the inner 

 end tapers into a fiber connected with a ganglion cell lying in the connective- 

 tissue layer ; the outer end may taper to a point or not. In the tentacles these 

 sensory cells are associated in groups, forming the touch-papillae to which refer- 

 ence has already been made. The gland cells are of the kind known as tubular 

 and always occur singly. 



