STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 195 



these spicules agrees with that of the subgastral sagittal 

 triradiates of Sycon and Grantia, but an apical ray is 

 developed which projects into the gastral cavity^ almost in a 

 line with the centrifugally directed basal ray. Hence these 

 spicules are not homologous with the subgastral quadriradiates 

 of some Araphoriscidse. their position being different. 



The articulate tubar skeleton of Grantiopsis cylindrica 

 is very remarkable^ owing to the peculiar form of the spicules 

 of which it is composed. These spicules are the most extreme 

 modifications of the sagittal triradiate type which I have ever 

 seen or heard of. They consist almost entirely of the strangely 

 developed centrifugally directed basal ray, which is straight, 

 fusiform, gradually sharp-pointed at the distal end, and at the 

 proximal end provided with a pair of minute, widely divergent, 

 conical teeth, which represent the extremely reduced oral rays. 

 The basal ray measures about 0-3 by 0*008 mm., while the oral 

 rays are only about 0*003 mm. long. The entire tubar 

 skeleton is made up of these spicules and of the basal rays of 

 the subgastral quadriradiates, arranged usually in single series, 

 but with overlapping ends, each series comprising only about 

 three spicules (fig. 11). 



Ute (figs. 12—14). 

 In this genus we find the skeleton arranged as in Grantia, 

 only with the addition of a number of very large oxeote 

 spicules disposed longitudinally in the dermal cortex. The 

 gastral skeleton is exactly like that of Grantia and Sycon, 

 and so also is the tubar skeleton in most species. In Ute 

 argentea, however, there is, as Polejaeff points out (8), a 

 tubar skeleton composed only of a single joint, i. e. of the rays 

 of the subgastral sagittal triradiates. To this tubar skeleton 

 the term " inarticulate " has been applied, but this appears to 

 me to be a mistake, for it is very different from the so-called 

 "^^ inarticulate" tubar skeleton of the Heteropidse and Am- 

 phoriscidae. As I shall show later on, however, the distinction 

 between ''articulate" and "inarticulate" tubar skeletons is 

 not altogether satisfactory. 



