208 



ZOOLOGY 



finally we have, in typical cases, a structure resembling a 

 hollow vase perforated with holes. The central cavity 



has an opening above called 

 the osculum (little mouth), 

 while the walls are perfo- 

 rated by pores. During life 

 water enters by the pores 

 and passes out through the 

 osculum. The inner cavity 

 is lined with peculiar flagel- 

 lated cells, the base of each 

 flagellum being surrounded 

 by a little cup or collar. 

 The whiplike movements of 

 the flagella cause the neces- 

 sary flow of water. Thus 

 the sponge, beginning life as 

 a free-living larva, as if on 

 the way to produce a rela- 

 tively high type of animal, 

 assumes a vegetative form, 

 and appears almost to lose 

 its integrity as an individual. 

 The cells of which it is com- 

 posed are less definitely asso- 

 ciated together than those of 

 the higher animals, so that 

 the distinction between a 

 sponge and an aggregate type 

 of protozoan is not so radical 

 From "Animate creation^ as might at first appear. In 



FIG. 46. Skeleton of Euplectella as- the Sponge, however, the Cells 



pergillum, or Venus-cup sponge, com- , , . . , . , 



posed of flinty fibers; about | natural ai ~ C DOt a11 Similar, Cither in 



form or function. 



size. 



