46 



IXVEKTEIiKATE ANIMALS. 



are termed the " pores " ; and though permanently i)resent in the 

 skeleton, they are only temporarily present in, the sponge - flesh, 

 being produced afresh, when recpured, as openings between the 

 sponge-particles which form the outer layer of the spimge. The 

 "pores "are the external openings of a series of canals which 

 ramify in every direction through the spcjnge, and which ai-e usually 

 called "inhalant canals," as it is through these that the outer water 

 is conveyed into the interior of the sjionge. The " inhalant canals " 

 idtimately ojien into a second series of cuials, which converge to 

 form one or more large tubes which open on the surface by a corre- 

 sponiliug number of large apertures. These large tubes carry the 

 water out of the organism again, and they are hence called "ex- 

 halant canals " (fig. 21), while their surface-openings are the"oscula." 



Fif^. 21^. — A, Arimlhi j^nj,ii„,i,l,:^, a ninnns simhi;,'!' slmwiii^^ (tpoul.T anil porrs. B, 

 Suranihii r.iliafa, a ralcan'iiiid s|inn^c, nilai^'eil, sluiwiiiu^ tin' single UMiiiinal oscu- 

 luiii. (AacrScliiiii.il.) 



Tin-, "osciila" are ]H'rnianeiit (fig. 2.'5, A), and are often jilaced on 

 i.'liiiMiicy-like clrvations. If there is only one oscnlum, it is jilaced 

 at Um; a|,c\ of the spiiiige (tig. 23, B). What we n.sually call a 

 Hpongi' may consist of only a .single excretory opening or "o.sculum," 



