122 T. IJIMA : HEXACTINELLIDA. I. 



entire thickness, of trabeculse in an irregular cobweb-like arrange- 

 ment. On the whole the trabecular cobweb of the ectosome is 

 somewhat denser than in the region below the dermal paratangen- 

 tials, i. e., in the subdermal region (see PL V, fig. 36). 



It is important to notice that in the trabecular cobweb of 

 the Euplectellid ectosome in general, the most superficially situated 

 trabecule, i. e., those delimiting the sponge periphery from the 

 external world, are often, but not invariably, expanded paratan- 

 gentially in a film-like or membrane-like manner. The gaps, or 

 the ' pores,' inclosed by such flattened trabeculœ are of a 

 more or less roundish shape and give to the layer itself the 

 appearance of a perforated membrane. This has been called by 

 F. E. Schulze the ' dermal membrane,'' and accordingly, the 

 dermalia, as being situated beneath that membrane, have received 

 the name of ' hypodermalia.' Misleading and inappropriate as 

 the latter appellation seems to me to be (see p. 46), the former 

 may with advantage be retained for the purpose of description. 



The dermal membrane then forms only a small part of what 

 I have called the ectosome in Euplectellidœ. Tn other families 

 in which the distal rays of the dermalia do not come into 

 development, the ectosome becomes, as suggested on p. 46, 

 greatly reduced in thickness in that the dermal membrane is 

 brought down to the level of the dermal paratangentials. It 

 is all the thinner because of the subdermal cavity which is 

 generally more spaciously developed in those forms than in 

 Euplectellidae. The dermal membrane then stands nearly or 

 quite by itself for the soft part of the entire ectosome, in 

 which case the two names may be considered as practically 

 synonymous. 



The ectosome of E. marshalH in particular requires a few 



