F. S. CONANT ON THE CUBOMEDUS.E. 39 



the nerve. It is evident that the tissues which elsewhere on the subum- 

 brella were differentiated into muscle epithelium and muscle fibre have 

 here become nerve epithelium and nerve fibre, a point that has not been 

 remarked upon before, so far as I remember, and that may be of interest 

 in connection with the iieuro-muscular theory. The epithelium of the 

 nerve (sen) is seen to be made up of cells whose inner ends narrow down 

 into a kind of stalk or process that runs to the gelatine of the supporting 

 lamella (gs) and there joins a little cone of the gelatine that juts out to 

 meet it. The cells are smaller in general than those that overlie the 

 muscle layer, especially on the two lateral margins of the nerve, where 

 they are more crowded together and overarch the nerve-fibres. The 

 fibres are seen in cross-section between the processes of the cells. They 

 apparently must lie imbedded in some clear, watery fluid that does not 

 show in the preserved material. The processes of the epithelial cells 

 give the fibres the appearance of lying in alveoli, or being divided into 

 strands, and one of these strands (ax) is always discernible among the 

 others by reason of its more numerous or finer or more compactly 

 massed fibres. This is the " axis " of Glaus. Here and there in its course 

 appear ganglion cells having their long axis in the longitudinal direction 

 of the nerve. Elsewhere, in the nerve as well, and usually nearer to the 

 surface, are found other ganglion cells, mostly bipolar, some multipolar, 

 which are readily distinguishable from those of the axis by the fact that 

 their long axis lies across the nerve. One of these cells is shown in the 

 figure (gc). Here and there in the epithelium alongside the nerve are 

 found mucous cells (me), distinguished by their clear contents and by the 

 small exhausted-appearing nucleus at the base with a few threads of 

 protoplasm. 



In Fig. 48 I have tried to represent the structure of the nerve by 

 means of a series of five different views such as would be given by 

 focusing at five successive levels. In the first (1) we have the epithe- 

 lium of the nerve (sen in Fig. 47) in surface view, the cells appearing 

 polygonal in outline, with here and there a mucous cell. In (2) we find a 

 very slight layer of ganglion cells and fibres having a transverse direction 

 (gc and fp in Fig. 47). These are continuous with the plexus of fibres and 

 ganglion cells which lie above the muscle layer all over the subumbrella, 

 and which represent the motor part of the nervous system. This connec- 

 tion with the nerve shows how co-ordination is effected. At the same 

 level are found fibres of the axis also having a longitudinal direction. 

 In (3) is seen the main body of fibres, divided in the osmic preparation 



