90 BRITISH SPONGES: 



in common with that of higher animals ; but it has some ana- 

 logy surely with that imbibition and influx of water into the 

 body of most radiated and molluscous animals which takes place 

 through the skin and through certain canals which Delle Chiaie 

 has described and distinguished as their aquiferous system. 

 The canals, in both cases, arc not vascular tubes with mem- 

 branous parietes but rather furrows excavated in the flesh 

 or substance of the body, and leading into wider channels 

 equally milined : they have in common a direct commimication 

 with the circumfluent water, which alone ever flows in them ; 

 and the entrance of this water seems to be in a great measure, 

 or entirely, independent of the will of the animals, but the poly- 

 pes and mollusca only have the power of expelling it when 

 they choose by the contraction and compression of the parts 

 which the canals traverse. There is, however, a wider dififer- 

 ence in the arrangement of the aquiducts in the two classes : — 

 in the radiata and mollusca the pattern is the same in every in- 

 dividual of each species ; but in the sponges it has no constancy, 

 so that in no two specimens of the same species do we ever find 

 the arrangement to be exactly alike. 



This inconstancy seems to prove that the direction of the 

 aquiducts through the sponge, and the position of their orifices 

 or oscula on the surface, is very much a matter of chance ; and 

 that their formation is the result of a mechanical cause liable 

 to be diverted from its course ■ by exterior circumstances. If 

 we follow the growth of a sponge we may feel still more con- 

 firmed in this view. The species begins as a spot-like crust 

 of uniform texture, porous throughout, and nearly equally so. 

 In this primitive homologous condition, there is nevertheless a 

 perfect circulation, — a current which seeks the interior and an- 

 other which flows from it to mix with the circumfluent medium. 

 As the sponge grow s in extent and depth the space for imbibition 

 is enlarged; and the centrifugal water, in its efflux, flowing at first 



