280 AKT. 7. 1. IJIMA : HEXACTINELLIDA, IV. 



AU the rest of the external siirftice is thickly covered with 

 large or moderately large pentactinic prostalia, forming a persistent 

 gossamer-like layer much as we have seen in R. unguiculatus. 

 The layer, which may be 9 mm. thick, is sufficiently resistent to 

 allow handling the specimens without touching the dermal surface. 

 The pentactins arise invariably in tufts, often ten or even more 

 in number to each, situated at intervals of 4-10 mm. from one 

 another. The ectosome projects at the tuft-basis in the form of 

 a narrow papilla-like prominence which may be 2 mm. high (PI. 

 XXII., fig. 12) ; and this prominence always occupies a position 

 central to each irregularly radiating group of hypodermal fibers 

 ( = paratangentials of hypodermal pentactins ; same plate, fig. 16). 

 In the prostal tufts the different paratangential heads usually lie 

 at various distances from the dermal surface, according to the 

 order in which their successive protrusions took place. 



In certain specimens I find in some places a film-like sub- 

 stance stretching in streaks and patches between the prostalia. The 

 presence of nuclei in it can not be demonstrated and tliere can 

 be little dou])t that it represents something — probably mucous — 

 foreign to the sponge. 



The dermal surface is rather uneven. As in all species with 

 diactinic dermalia, the dermal meshes are observed under the lens 

 to be irregularly shaped, — not quadrate. The layer is supported 

 by the intersecting hypodermal fibers already referred to (fig. 



16). 



There is a comparatively wide subdermal space, traversed by 

 the shafts of hypodermalia and prostalia and also by the distal 

 ends of certain parenchymalia. On sections of the dried body- 

 wall it plainly presents itself to the unaided eye as a layer which 

 may be 1-2 mm. wide. 



