16 



plasm. This protoplasm in the living sponge is differentiated 

 into two layers — an inner of ciliated cells, and an outer layer 

 named the synctium, in which the cell structure in the ordinary 

 condition is not recognizable. It is in this synctium or exoderm 

 (outside skin) that the skeleton is secreted. This skeleton may 

 be either keratose or of horny fibre, as in the sponges of com- 

 merce, and composed of a substance, which, in its ultimate 

 analysis, much resembles silk ; of carbonate of lime, or 

 of silica. 



Thus there are three great divisions of sponges 



1 Keratose, 



2 Calcareous, 



3 Siliceous. 



With the first two we are little concerned in our present 

 inquiry. It is the siliceous one which demands our attention, but 

 there are some points common to all three classes which it will 

 be necessary to touch on. The sponge in its natural state is a 

 very different body from that with which we are familiar. The 

 skeleton of a man would hardly give to one unacquainted with 

 humanity an adequate idea of the living creature. It is so with 

 the sponge. In life then, this skeleton is clothed with a pellucid 

 semi-transparant gelatinoid substance called sarcode, very like 

 white of egg. It is variable in colour and insoluble in water. 

 A specimen of an ordinary commercial sponge when placed in 

 spirit directly it is taken from the sea has the whole of the 

 interior nearly as solid and firm as a piece of animal liver. 



This jelly-like matter is traversed by innumerable canals. 

 " If viewed with a lens under watei*," says Mr. Gosse, " while 

 in a living state they display vigorous currents constantly pour- 

 ing forth from certain orifices, and we necessarily infer that the 

 water thus ejected must be constantly taken in through some 

 other channel. On tearing the mass open we see that the whole 

 substance is perforated in all directions by irregular canals lead- 

 ing into each other, of which some are slender, and communicate 

 with the surface by miuute but numerous pores, and others are 

 wide and open by ample orifices ; through the former the water 

 is admitted, through the latter it is ejected." Instead of an 

 excurrent system of canals there are in some orders great cloacal 

 cavities extending from the top to the bottom of the sponge, 

 which receive the water and excrementitious matter. Enveloping 

 the entire mass of the sponge is the dermnl membrane, pierced 

 by the orifices of incurrent or excurrent canals. 



From this short general view of the structure and physiology 



