SPONGID^. 33 



nominated ^oscula,' the smaller orifices being spe- 

 cially distinguished as the * pores.' 



On cutting open the Sponges, the oscula and 

 pores are seen to be connected each with its proper 

 system of ^ canals.' What are termed the ' ex- 

 current ' canals proceed immediately from the os- 

 cula, and, after forming a somewhat complicated 

 network within the outer layer of the Sponge, 

 finally communicate with another system of * in- 

 current' canals, which, in their turn, terminate 

 in the Spores.' Both of these sets of canals 

 are produced by the peculiar arrangement of the 

 horny fibres which compose the skeleton, and 

 in the living animal are invested on all sides 

 by a coating of the glairy gelatinous sarcode. 



If a fragment of living Sponge be placed, in a 

 watch-glass, on the stage of a microscope and ex- 

 amined with a low magnifying power, a curious 

 spectacle will, under favourable circumstances, 

 come into view. Currents Tvill be seen to issue 

 rapidly from the oscula, whilst at the same time 

 water is being continually absorbed by the pores. 

 In this manner a sort of circulation is maintained 

 within the two systems of canals which connect 

 the oscula and pores with one another. The 

 currents are rendered more readily observable, by 

 diffusing finely powdered indigo or carmine in the 

 water containing the specimens under examina- 

 tion. 



When the foregoing phenomena were first 

 noticed (in 1827) by ^^' Cri'ant, they excited much 

 attention among naturalists, though for a long 

 time afterwards the mechanism by which they 

 were effected, remained altogether undiscovered. 

 More recently the subject has been investigated 



D 



