ACANTHASCIN^. 129 



on this account deserving perhaps to be erected into a new and 

 distinct genns. It frequently happens in firmly fixed forms that 

 the basal region is bent to a greater or less degree ; this prob- 

 ably results from the sponge growing on an inclined or vertical 

 substratum while the main part of the body stands erect, direct- 

 ing the osculum upwards (PI. XIV., fig. 14 ; PL XVIL; etc.). 



The l^ody is often laterally compressed, so that we may 

 speak of the median sagittal plane that divides the body into 

 symmetrical lateral halves. The bending of the sponge-base, 

 whenever it occurs with a laterally compressed body, is invariably 

 in this plane (see PI. XVIL; etc.) ; so that the bent base is 

 subjected to the same lateral compression as that of the main 

 body, — a rule, the applicability of which is not restricted to the 

 Acanthascinœ alone but extends to the Possellidse in general. 



Many species have the power of opening secondary oscula 

 on the general body-wall or of budding out daughter persons, 

 although neither of them are ever formed in any great number. 

 Some species seem to be more prone to forming them than others. 

 A daughter person first arises as a cœcum-like outbulging of the 

 wall, eventually to open an osculum at the summit. In the case 

 of the laterally compressed mother body, it may be given as the 

 rule that the bud formation takes place on or along the median 

 edge, generally in the lower part (PL XVII.; PL XX., fig. 2., 

 etc.). 



As to the spiculation, the ]X(^^enchymalia are always and 

 exclusively diactins. Hexactins never occur amongst them. When 

 a number of the diactins are combined into bundles, some of 

 them may be distinguished from the rest as the ijripncialia on 

 account of their greater strength. The principalia are elongate 



