42 I. IJIMA : HEXACTINELLIDA. I. 



has called tlie dermal and the gastral membrane are structurally not 

 distinguishable from the more deeply situated trabeculse. Thej'' 

 belong, in my opinion, within the pale of these, differing only in 

 their being somewhat membranously expanded in relation to their 

 position at the limiting surface. (See anon under E. marshaUi). 



The outer surface of the choanosome in Euplectella, visible 

 through the delicate cobweb-like ectosomal layer, is perforated 

 by small apertures (ostia Eauff) leading into the incurrent canals. 

 The excurrent apertures (postica Rauff), which are on the whole 

 very much larger, open as already indicated freely on the gastral 

 side, being not covered over by a continuously developed 

 endosomal layer. 



Spiculatiox. — The triaxial spicules of Hexaetinellids in 

 general may primarily be separated into two groups, which mor- 

 phologically are sharply defined. One of these comprises the 

 hexasters or rosettes with their manifold Varieties and modifica- 

 tions ; while in the other should be put all the rest of the 

 spicules — the hexactins and their direct derivatives by the sup- 

 pression of one or more rays — which present on the whole much 

 simpler and more primitive characters. 



Hexactins and their direct derivatives. — These furnish par 

 excellence the supporting spicules of the body. They fall under 

 several categories, such as the parenchymalia, the dermalia, &c., 

 according to different topographical and functional circumstances. 



The parenchymalia, which term I use in a sense differing 

 from F. E. Schulze's in so far as not to include the hexasters, 

 have their seat in the choanosome. The more strongly developed 

 spicules in this category are known as the princijmlia, in dis- 

 tinction from which the remaining parenchymalia of weaker 

 development and somewhat subsidiary function may be designated 



