STUDIES (5n the comparative anatomy of sponges. 193 



ance from the point of view either of the morphologist or the 

 systematist. 



Sycantha. 



The skeleton of this genus appears to conform exactly to the 

 normal Sycon type, which appears to favour my view that 

 the canal system is but a slight modification of the Sycon 

 type. 



Grantia (figs. 9, 10). 



In Grantia we find the skeleton built upon the same 

 essential plan as in Sycon, but there is, in addition, a well- 

 developed dermal skeleton lying in the dermal cortex, which 

 covers over the ends of the radial chambers and inter-canals, 

 while we no longer find each chamber surmounted at its distal 

 extremity by a tuft of uniaxial oxea. The gastral skeleton 

 and the articulate tubar skeleton are precisely similar to what 

 we found in Sycon, as will be at once evident on referring 

 to fig. 9, representing the anatomy of Grantia extus- 

 articulata. 



The skeleton of the dermal cortex in this genus consists 

 principally of triradiate spicules lying parallel to the dermal 

 surface. These spicules may be sagittal, with the basal ray 

 directed, as in the gastral cortex, towards the base of the 

 sponge. A good example of this arrangement is seen in 

 Grantia labyrinthica (9). 



In addition to the triradiate spicules we also find in the 

 dermal cortex of Grantia, in most if not all species, a number 

 of oxeote spicules placed at right angles to the dermal surface. 

 These may either be very small and numerous, as in Grantia 

 extusarticulata (fig. 9), or they may be large and fewer, as 

 in G. Vosmaeri (fig. 10). In the former case they form a 

 kind of crust, and are spoken of by some writers as " mortar- 

 spicules : " their inner ends only penetrate the dermal cortex 

 for a short distance. In the latter case their inner ends may 

 penetrate through nearly the entire thickness of the sponge 

 wall. In no case in this genus do they form tufts at the ends 



