14 ARTHUR DENDY. 



The Septum. — The histological structure of the septum 

 which divides the main gastral cavity from the cavity of 

 the float is practically identical with that of the inner walls 

 of the endodermal canals^ with which it is directly continuous. 

 On its lower face it is covered by a layer of lightlj- staining 

 cells with small nuclei and finely granular contents, and this 

 is separated by a moderately thick layer of mesogloea from 

 the finely granular syncytium which covers its upper surface. 

 Some of the supporting membranes of the float are attached 

 to its upper surface, and probably originate from the septum 

 in the same way as those already described originate from 

 the inner walls of the endodermal canals. 



Tentacles of the Float. — The tentacles of the float are 

 histologically identical with the large tentacles of the pro- 

 boscis, as will be seen by comparison of fig. 15 with the 

 description already given. The peculiar manner in which the 

 axial tissue seems to project into the cavity of the float in the 

 form of a cushion or plug has already been referred to. In 

 the projecting plug, however, when best developed, the 

 network of tissue is made up chiefly of a finely granular 

 frothy syncytium, with very little mesogloea and small nuclei. 

 In the tentacle itself the granular material is hardly recog- 

 nisable, the septa (fig. 15, S.M.T.) are very thin, and the 

 nuclei (fig. 15, Nu.) much larger and of a different character, 

 like those in the proboscis tentacles. Thus the ^^ plug " 

 seems to be to some extent transitional in character between 

 the true axial tissue of the tentacle and the very much coarser 

 reticulation formed by the supporting membranes in the 

 interior of the float. It is not always recognisable as a 

 distinct structure, however, and even where best developed 

 it passes gradually into the axial tentacular tissue beyond, 

 while its apparent histological differences may be in part due 

 to the want of penetration of the osmic acid with which the 

 specimen was hardened. 



The endodermal canals come very close to the bases of the 

 tentacles, and we may be pretty certain that the axes of the 

 latter are endodermal in origin, though, as in the case of the 



