8 THE CTENOPHORA 



cord, from which muscular slips are given off to the accessory 

 filaments. The tentacle itself is a solid, muscular, and exceed- 

 ingly extensile filament (Fig. III. 10). The accessory filaments 

 are simple and thread-like in Pleurobrachia, but in Hormiphora 

 certain of them are thickened and furnished with digitiform 

 appendages which, from their supposed resemblance to a minute 

 Eolis, are often called eolidiform appendages. The whole surface 

 of the tentacle and its accessory filaments is covered by densely 

 crowded " lasso-cells," structures characteristic of the Ctenophora, 

 which will be described in detail further on. 



The musculature of the Ctenophora is wholly derived from 

 the mesoblast, and there are no epithelio- muscular cells. The 

 muscle fibres are for the most part much branched, and are not 

 grouped into bundles except at the bases of the tentacles, in the 

 tentacles themselves, and in the regions of the mouth and aboral 

 sensory organ, where they form sphincters. There is a well-marked 

 layer of musculature under the body wall, consisting of an external 

 layer of longitudinal, and an internal layer of transverse fibres. A 

 similar musculature invests the stomodaeum and the gastrovascular 

 canals. The gelatinous substance of the body is traversed by 

 numerous fibres, whose general direction is radial, from the stomo- 

 daeum and gastrovascular system to the body wall. 



The histology of the Ctenophora has been carefully studied by 

 Samassa (21), to whose paper the reader is referred for details. 

 The epithelium of the body is peculiar, being formed of large gland 

 cells lying in an interstitial tissue, in which many nuclei, but no 

 cell boundaries, are to be distinguished. In the neighbourhood of 

 the aboral sense organ, the ciliated ridges and the costae, the gland 

 cells become smaller and less numerous, and the interstitial tissue 

 is replaced by a simple cubical epithelium. The most characteristic 

 histological feature of the Ctenophora is the presence of the 

 lasso-cells (Fig. III. 1, 2). Each lasso-cell has the shape of a 

 hemispherical cup, the convexity turned outwards and covered 

 with minute sticky papillae. To the inner concave side are 

 attached two filaments : the one an exceedingly fine central proto- 

 plasmic thread, in the upper part of which a much attenuated 

 nucleus can generally be distinguished. The other is a contractile 

 fibre thicker than the first, attached like it to the centre of the 

 convex surface of the cup, and coiled in the first*part of its course 

 in a close spiral. Eventually the spiral thread tapers off into a 

 fine filament, which, according to Chun, is attached to the muscle 

 fibres forming the axis of the tentacle. The lasso-cells lie close 

 together, forming a complete investment for the tentacle, with only 

 very sparse interstitial tissue between. When any foreign body 

 comes into contact with the tentacle, the lasso-cells adhere to it 

 by their sticky convex surfaces, are withdrawn from the surface, 



