240 PERCY SLADEN TRUST EXPEDITION 



is very characteristic, the surface being beset with irregularly arranged, more or less 

 elongated conuli. These conuli are stiff and supported by spicule bundles. In addition 

 to the conuli there are short, root-like attaching processes on the lower surface. Between 

 the conuli the surface is in many places minutely reticulate, with pore-sieves in the meshes 

 of the reticulation. There are no conspicuous vents, and it appears probable that the 

 exhalant openings are covered over by sieve-nets. The smallest specimen is almost 

 spherical, but the two larger ones are both slightly flattened dorsoventrally, and the 

 largest (fig. 7) has a rather prominent equatorial ridge, dividing the more flattened upper 

 from the more convex lower surface ; on this ridge the conuli are especially numerous. 

 The colour in spirit is dark purplish-brown. All the specimens show clearly the small 

 white specks due to the accumulation of trichodragmata just beneath the surface, as 

 described by Thiele. The texture is firm and compact. 



The cortex, in the largest specimen, is about 3 mm. thick. It is divided into two 

 very distinct layers, an outer, soft, pigmented one, to which the colour of the sponge is 

 due, and an inner, very dense, fibrous one with only occasional pigment cells. The inner, 

 fibrous layer is very much more strongly developed than represented in Thiele's figure, 

 the difference being no doubt correlated with the much larger size and presumably greater 

 age of the specimen. It makes up considerably more than half the thickness of the 

 cortex and is composed of an extremely dense interlacement of fibre-tracts running in all 

 directions. From its outer surface fibre-tracts run into the outer layer of the cortex, 

 where they form a loose network, concentrated, however, towards the surface and around 

 the radially arranged bundles of megascleres. In the meshes of this network lie the large 

 vesicular pigment cells described by Thiele, the pigment being most strongly developed 

 in the deeper part of the layer. The outer layer of the cortex also contains great rounded 

 masses of trichodragmata. 



The inner layer of the cortex is pierced by narrow canals, which may branch and 

 anastomose with one another and frequently unite to form wide chones, and whose course 

 is clearly marked out by the microrhabds lying in their walls. In the outer layer of the 

 cortex the canals are not easy to follow, but they appear to be still narrow and no doubt 

 lead inwards from the dermal pores. 



Beneath the fibrous layer of the cortex there is a much thinner coUenchymatous layer 

 containing numerous subcortical crypts. This layer also contains a good many pigment 

 cells. 



I am unable to say anything definite about the exhalant canal system, but there is 

 good reason to believe that it is similar to that described by Topsent [1894 g] for his 

 Sanidastrella coronata, a sponge which is obviously nearly related to our species. In 

 that sponge the exhalant canals open at the sides of the much elongated dermal 

 appendages (corresponding to the conuli of Rhabdodragma), not by large openings but 

 through sieves. At the same time it seems probable that in our species the exhalant 

 openings are not confined to the conuli. 



The skeleton of the choanosome is very dense, consisting of pretty closely packed, 

 radially arranged, large oxea and plagiotrisenes. Most of these terminate below the cortex, 

 but here and there, at wide intervals, dense bundles of oxea and plagiotriasnes penetrate 



