46 Memoirs of the Indian Museum. [Voi,. V, 



or inclined plane and, probably after branching at least once, ultimately reach the 

 small, ill-defined lacunae round which the ciliated chambers are arranged. The 

 chambers are oval in outline and about 0-0026 mm. long by o - oo2 mm. broad. Fine 

 exhalent channels run from the lacunae and, after combining once or more, reach the 

 superficial canals; their course is naturally upwards in a sloping direction. The soft 

 parts of the sponge may be described as compact in comparison with those of other 

 species of the genus. 



The skeleton consists of two distinct parts : plumose spicule-fibres that terminate 

 in free brush-like bunches of spicules on the surface of the sponge, and a basal horny 

 membrane containing isolated spicules, which are as a rule smaller and more slender 

 than those of the fibres. 



The spicule-fibres differ but slightly as a rule from the type characteristic 

 of the genus.- Sometimes, however, at the extreme margin of growing sponges and 

 in stunted specimens they resemble the simple upright bunches of Prosuberites. 

 In those sponges that may be regarded as fully formed and well developed the 

 primary fibres are directed for a short distance vertically upwards from their base, 

 then bend over gradually and run for some distance horizontally (that is to say 

 parallel to the surface of the sponge) , and finally protrude upwards. The spicules 

 all point away from the basal extremity of the fibre. Except at the base, I have 

 not been able to detect any binding substance in the fibres, and the extreme readi- 

 ness with which the whole structure is disorganized by the exercise of pressure 

 would seem to prove that substance of the kind is not more, at most, than very 

 scantily present. The primary fibres terminate in the usual manner in bunches of 

 spicules directed outwards as well as upwards ; the tips pierce the dermal membrane. 

 The whole disposition of the skeleton is closely correlated with that of the osculum 

 and exhalent channels, and, indeed, with that of the water-system generally. The 

 direction of the spicules is away from the osculum, so that the fibres they form 

 radiate outwards from it between the main exhalent channels, parallel to which they 

 run. At the margins of these channels the primary fibres give off short lateral 

 branches that have a somewhat fan-like arrangement individually and lie in the 

 dead sponge horizontally and practically parallel to the surface. Those from the 

 primary fibre on either side of the channel nearly meet in its middle. When, how- 

 ever, a current of water passes through towards the osculum it raises these lateral 

 fan-like branches and causes them to arch over immediately under the dermal 

 membrane, to which they give support. The tips of their spicules, which are 

 directed outwards from the primary fibre, do not penetrate it as do those of the 

 spicules which form the terminal brushes. Single spicules lie scattered in a horizon- 

 tal direction on the floor of the superficial channels. 



At the base of the sponge there is a delicate but distinct horny membrane in 

 which, as already stated, spicules usually smaller than those of the fibres lie 

 scattered horizontally. This structure is very liable to be overlooked as it can be 

 separated from the stone to which the sponge is attached only with some difficulty. 



AH the spicules are normally tylostyles and there are no microscleres. The 



