10 AlîT. ]. 1. T.TIMA : IIEXACTINELLTDA, III. 



ing a rudimentary gastral cavity, is to be ascribed to the species. 

 If that be true, the species should be said to agree with Placosoma 

 imradiclyum not only in the excessively thick development of 

 the originally cup-like sponge-wall but also in the possession of 

 numerous secondary oscula in addition to the single primary one. 

 The development of the frontal lattice and the occurrence of 

 oscula exclusively on the opposite side of the body in Placosoma 

 paradicfyum i:)robably stand in relation to the physical circum- 

 stances of the habitat favoring the ingress of water on the one 

 side or its egress on the other, — possilily in that the former is 

 exposed to, and the latter sheltered from, a f>i'evailing current. 

 Here again we seem to have to do with a structural arrangement 

 which does not stand quite isolated among the Hexactinellida. 

 So, for instance, in Semper ella schuUzei, as is well known, the 

 external (dermal) surface is differentiated into two sorts of tracts, 

 the afferent and the efferent. The latter may indeed be said to 

 occur here on all sides of the columnar body, but is relatively 

 nuich more extensively developed on its one side than on the 

 other, — a fact, which seems to have hitherto escaped the attention 

 of observers, but which I liave found to hold good for all the 

 specimens examined by me. A closer analogy to the condition 

 that obtains in Placosoma paradiclyuin is apparently exhibited by 

 Poliopogon amadou, as known to me through F. E. Schulze's 

 representations in the Challenger Report. In this form the entire 

 external surface is, in my opinion, to be regarded as dermal, the 

 gastral surface being nowhere outwardly exposed. The discharge 

 of water is effected through orifices which are situated exclusively 

 on the one — the concave — side of the half-rolled lamella-like 

 body. This concave surface has been called by F. E. Schulze 

 gastral, which appellation however seems inadmissible in view of 



