E. MAESHALLI. — GEN. ARRANGEMENT OF SOFT PARTS. 121 



or, wluit amounts to the same thing, by the comparative sparse- 

 ness of trabecuUxî. The hicunse in this region are known as 

 the subdermal cavity (PI. V, fig. 36, s.o.). It is from this 

 cavity that the incurrent canals appear to arise, thence to pene- 

 trate into the choanosome. Thus, generally speaking, the ectosome 

 extends itself over, and is separated from the choanosome by, the 

 subdermal cavity ; the paratangential rays of the dermalia serve 

 as its main support, while the proximal rays of the same as well 

 as a variable quantity of subdermal trabeculœ, effect its connec- 

 tion with the choanosome. 



As it presents itself in Euplectellidse, a considerable thick- 

 ness is to be ascribed to the ectosome, a fact which is apparently 

 caused by the presence of well-developed distal rays to all the 

 dermalia. Each of the rays just mentioned stretches out the 

 thickness of the ectosome in distal direction so as to form a 

 minute conulus on the external surface of the sponge. The 

 boundary delimiting the ectosome from the underlying subdermal 

 stratum is about as ill-defined as can be. This is in a great 

 measure due to the fact that the subdermal cavity never reaches 

 a prominent degree of development in spaciousness, a peculiarity 

 which stands in correlation both with the fact that numerous 

 proximal rays of the dermalia traverse the region at comparatively 

 short intervals, and also with the small caliber of the incurrent 

 canals. Nevertheless, I think there are grounds for considering 

 that the plane of the dermal latticework (paratangential rays of 

 the dermalia), which in Euplectellidse lies, as is well known, at 

 a certain distance below the external surface, indicates in a general 

 way the boundary between the two strata. The ectosome is there- 

 fore not to be described as a perforated plate-like layer, but, as a 

 part of the general trabecular system, it consists, throughout its 



