14 ON CLASSIFICATION. 



are all conjoined in a single layer, so as to form a continuous 

 lamellar membrane, which constitutes the outer and superficial 

 laver of the bodv. Beneath this is a wide cavity, communi- 

 eating with the exterior by means of minute holes in the super- 

 ficial layer (b), and, of course, filled with water. The cavity 

 separates the superficial layer of the sponge from its deeper 

 substance, which is of the same character as the superficial layer, 

 being made up of a number of aggregated sponge particles, each 

 of which has a nucleus and is competent to throw out numerous 

 pseudopodial prolongations if detached. While the living sponge 

 is contained in water, a great number of currents of water set in 

 to the wide cavity beneath a, a, through the minute apertures 

 (b), which have thence been termed " inhalent." 



In the floor of the cavity, there are a number of apertures 

 which lead into canals ramifying in the deep layer, and eventu- 

 ally ending in the floors of certain comparatively lofty funnels, or 

 craters. The top of each of these presents one of those larger 

 and less numerous apertures, which have been referred to as 

 existing on the surface of the sponge, and which are fitly termed 

 " exhalent " apertures. For, as Dr. Grant discovered, many years 

 ago, strong, though minute, currents of water are constantly 

 flowing out of these large apertures ; being fed by the currents 

 which as constantly set in, by the small apertures and through 

 the superficial cavity, into the canals of the deeper substance. 

 The cause of this very singular system of currents remained for 

 a long time unknown. It was rendered intelligible by Dr. 

 Bowerbank's discovery of the existence of vibratile cilia in the 

 genus Gvantia, but it is only quite recently that the precise 

 nature of the arrangement of the apparatus which gives rise to 

 these currents in ordinary sponges, has been made out by 

 Lieberkuhn and by Carter. The canals which enter the deep 

 substance of the sponge become dilated into spheroidal chambers, 

 lined with sponge particles (Fig. 3, A, c, C), each of which is 

 provided with a vibratile cilium ; and as all these cilia work 

 in one direction — towards the crater — they sweep the water out 

 in that direction, and its place is taken by fresh water, which 

 flows in through the small apertures and through the super- 

 ficial chamber. The currents of water carry alone: such mat- 



