F. S. CONANT ON THE CUBOMEDUS^E. 43 



of the subumbrella from the surface of the bell cavity (sc) to the base of 

 the stalk hanging in the sensory niche (sn). One of the ganglion cells 

 (gc) that accompany the nerve is seen to have two nuclei, a not infre- 

 quent occurrence which has been pointed out by others. 



The same figure shows that the axis (ax) of the nerve has penetrated 

 the gelatine with the other fibres. Here at the base of the stalk it takes 

 a horizontal course and becomes directly continuous with the similar 

 structure of the other root, as Wilson, I believe, first pointed out. This 

 part of the nervous tract which runs horizontally along the base of the 

 stalk between the two roots (Fig. 49, rns) has been considered by Glaus 

 the representative in Charybdea of the upper nerve ring of the Craspedota, 

 which therefore exists in Charybdea in four separate portions. Seeing, 

 however, that the region in which it is found belongs to the subumbrella, 

 the homology seems very doubtful. Moreover, the fact that the axis of 

 the nerve ring runs through this outer portion, instead of remaining on 

 the inner surface of the subumbrella and passing to the radial ganglion, 

 rather indicates that the outer portion is part of the original course of 

 the nerve ring, while the portion that remains on the inner surface is 

 perhaps a later formation. 



A very interesting feature of the nervous system occurs in the same 

 region in the form of a tract of fibres underlying the endoderm, and 

 separated from the other fibres by the gelatine of the supporting lamella. 

 It is seen in vertical section in Fig. 52 (ef), which is a section through 

 the base of the stalk in just about its median plane, and, therefore, to one 

 side of the arrow w-z in Fig. 49 and the corresponding drawing, Fig. 51. 

 In cross-section it is represented also in Fig. 50 (enf). It varies in size 

 and prominence very much in different specimens. Fig. 52 is a camera 

 drawing of it in the case that showed it most developed. Ganglion cells 

 are found in it, but comparatively infrequently. In some cases the tract 

 itself can hardly be found with certainty. Hesse has described in a 

 Rhizostome a much more highly developed tract in a corresponding 

 position on the base of the marginal body. Fibres from the " outer 

 sensory pit" pass through the gelatine to the -sub-endodermal tract, 

 which is described as surrounding the epithelium of the canal of the 

 marginal body like a collar and is most thickly developed on the under 

 surface of the canal, at the place that just corresponds with the point 

 where, and where only, I find the tract in Charybdea. Hesse thinks that 

 fibres then pass from this region to the nervous epithelium of the " inner 

 sensory pit" lying underneath the base of the marginal body, which 



