234 CLASSIFICATION OF ANIMALS 



the capture of food particles as in the Choanoflagellata and 

 digestion is Kkewise intra-cellular. 



521. In more complex sponges the flagellate epithelium is 

 limited to certain depressions of the cloacal surface which form 

 chambers radiating from the cloacal cavity. These are called 

 flagellate chambers or radial canals. In many cases the 

 flagellate chambers are so far removed from the cloaca that 

 another system of canals results, the excurrent canals, which 

 connects the flagellate chambers with the cloaca. In still 

 more complex forms the incurrent and excurrent canals are 

 branched. 



522. Sponges have no power of locomotion and only in some 

 cases can any evidence of contraction be observed directly. 

 However, the minute pores can be closed by the contraction of 

 the muscle cells of the mesoglea. Some sponges are quite soft, 

 almost jelly like, but usually the mesoglea is so filled with cal- 

 careous or silicious spicules as to render the sponge firm or even 

 hard. In some sponges the mesoglea contains a skeletal struc- 

 ture composed of horny fibres, in addition to the spicules. 

 This is notably the case with the common bath sponge in which 

 the spicules are not well developed. 



523. Sponges reproduce by a process similar to budding. 

 A fragment of the sponge separates, is carried away by water 

 currents, becomes attached and develops into a new sponge. 

 The sexual method is, however, the more frequent. The eggs 

 and sperm are developed in the mesoglea, where the egg be- 

 comes fertilized and begins its development. It escapes as a 

 ciliated larva, swims away and becomes attached with the 

 gastrula mouth down. The gastrula cavity becomes the 

 cloaca and an osculum is formed by thinning of the wall at the 

 end opposite the point of attachment. The incurrent pores 

 are formed in like manner. 



Order i. — The Calcispongiae have calcareous spicules of one, three or 

 four rays. Grantia. 



