46 



INVERTEBRATE ANIMALS. 



are termed the " pores " ; and though permanently present in the 

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

 being produced afresh, when required, as openings between the 

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

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

 ramify in eveiy direction through the sponge, and which are usually 

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

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

 ultimately open into a second series of canals, which converge to 

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

 sponding 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.'' 



Fig. 23. — A, Axinella polypoides, a fibrous sponge showing oscula and pores. B, 

 Sycandra ciliata, a. calcareous sponge, enlarged, showing the single terminal osca- 

 luin. (After Schmidt.) 



The " oscula " are permanent (fig. 23, A), and are often placed on 

 chimney-like elevations. If there is only one osculum, it is placed 

 at the apex of the sponge (fig. 23, B). What we usually call a 

 sponge may consist of only a single excretory opening or " osculum," 



