STUDIES ON THE COMPARATIVE ANATOMY OF SPONGES. 203 



dermal skeleton is made up principally of subregular tri- 

 radiates of various sizes, placed horizontally, but with no 

 definite arrangement; amongst these very minute slender 

 oxea are scattered, especially numerous around the osculum.^ 

 Around the main exhalant canals is a layer of small sagittal 

 triradiates, forming what must probably be regarded as the 

 gastral skeleton. No quadriradiates are present, and I have 

 not detected any special subgastral sagittal triradiates. The 

 skeleton of this sponge bears much the same relation to that 

 of Grautessa as that of Leucandra does to that of 

 Grantia. 



Vosmaeropsis Wilsoni is remarkable for the enormous 

 development of the cortex and its contained skeleton on both 

 gastral and dermal surfaces. The skeleton of the chamber 

 layer is like that of Vosmaeropsis macera (fig. 19), but the 

 sagittal triradiates are of unusually large size. A well- 

 developed oscular skeleton, in the shape of a fringe of oxeote 

 spicules, is also present, but this is not a feature upon which 

 much stress need be laid in any case. 



Heteropegma (fig. 20). 

 We have seen how, in Grantessa, the very characteristic 

 skeleton is derived from the ordinary Grantia type by the 

 mere addition of a layer of subdermal sagittal triradiate spicules. 

 The skeleton of Heteropegma is also derivable from the 

 Grantia type by an analogous change, only the subdermal 

 spicules are quadriradiates and not triradiates. Moreover it is 

 important to observe that the subdermal quadriradiates of 

 Heteropegma and other Amphoriscidse are not modifications 

 of subdermal sagittal triradiates, such as are found in the 

 Heteropidse. If they were so, we should expect to find the 

 basal ray still pointing inwards towards the gastral cavity, and 

 the additional apical ray lying in a plane more or less parallel 

 with the dermal surface. As a matter of fact, however, the 

 three facial rays of the subdermal quadriradiates lie parallel 



* Oxeote spicules are sometimes developed around the oscujum when tbej 

 cannot be found anywhere else in the sponge. 



