40 



Memoirs of the Indian Museum. 



[You. V, 



Only obscure traces of vertical fasciculation can be detected in the basal parts 

 immediately above this mass, but on the surface of the sponge the vertical tufts of 

 spicules so characteristic of several genera of Suberitidae are well developed. They 

 arise directly from vertical, entirely non-plumose columns in which all the spicules 

 point directly upwards and which can be traced downwards to different levels in 

 the sponge, some of them for at least one- third of its depth. The section figured on 

 pi. iv passes through the outer wall of a large horizontal canal probably belonging to 

 the exhalent system and it will be readily seen that the spicules here lie horizontally 

 parallel to one another. Had the section passed outside the canal altogether no such 

 horizontal stratum would have been shown, and had it passed through the lumen 

 instead of the wall there would of course have been a longitudinal gap. In the 

 former case it would have been possible to trace the vertical columns much further 

 down, and they would have merged gradually into the confused intermediate zone 

 of the skeleton instead of being sharply divided from it by a horizontal layer. 



Fig. 7. — Suberites sericeus, Thiele. 

 Vertical section through a sponge in phase A. with gemmules, x 150. 



If fragments of this confused intermediate layer be removed from the sponge 

 they will be found to be surprisingly coherent ; it is even possible to macerate the 

 flesh from them and preserve them intact. The explanation lies in the fact that many 

 of the spicules are cemented together in slender and often irregular fascicles by a 

 scanty but strong secretion of transparent horny substance and that both the fasci- 

 cles and single spicules not included in them are fastened to one another in a similar 

 manner at the points at which they impinge This is particularly noteworthy in the 

 neighbourhood of foreign bodies such as the stems of Hydrozoa (Bimeria) that lie 

 buried in the sponge. The heads of many spicules are embedded in a horny secretion 

 covering such bodies in exactly the same manner as at the base of the sponge and 

 these spicules seem to form as it were a nucleus from which the reticulation of the 

 skeleton arises (pi. iv, fig. 4a). 



