132 ART. 7. — I. IJIMA : HEXACÏINELLIDA, IV. 



ill tufts and the paratangential rays of their heads form a 

 gossamer-like veil at a certain distance from the dermal surface, 

 exactly as is known also in some Rossellids outside of the 

 Acanthascinœ. It is the oldest pentactinic hypodermalia that are 

 thus shifted out ; therefore, in Rhahdocalyptus the j^^n'^tangentials 

 of pentactinic prostalia are always found to be pronged. 



It may here be remarked that the basal anchoring spicules 

 of R. pl'umodiffiiaius Kiekp. ('oi) re])resent in all probability a 

 special adaptation of the pentactinic prostalia of the species. 



On the inside of the sponge-wall, hyj)ogastral strands are 

 usually more or less distinctly observable. They are to all 

 appearance nothing else than certain parenchymal bundles that 

 have dissociated themselves to a certain degree from the parenchy- 

 mal mass and have entered into the su])port of the endosomal 

 layer. They are seen to pursue a sinuous course and to intersect 

 one another at irregular intervals. 



Turning back to the external surface, the dermalia are small 

 rough-surfaced spicules which may be pentactins, stauractins or 

 straight diactins. .Vll the three forms or any two of them, the 

 number of whose rays are consecutive, may occur together in a 

 species, but it is usual that one or the other of the said forms 

 predominates to a greater or less degree. Hexactins, tauactins, 

 orthodiactins and monactins are (piite rare and exceptional, if 

 any of them occur at all among the dermalia- as they do in some 

 species. Pentactinic dermalia have the unpaired ray always 

 directed proximad. When stauractins or pentactins constitute 

 the main elements of the dermal layer, their cruciate paratangen- 

 tials are so arranged as to bring about a delicate latticework with 

 more or less regularly quadrate meshes ; whereas, in species with 

 diactinic dermalia the meshes formed are triangula]-, trapezoidal 



