THE CCELENTERATES AND SPONGES 139 



rock; many of these forms are also conspicuously colored 

 and present a marked contrast to the transparency, or the 

 inconspicuous colors, of most of the unprotected animals 

 of the open sea. 



The Ccelenterates known as Ctenophores, or comb- 

 bearers have a fairly pronounced bilateral symmetry. 

 They swim by means of eight rows of comb plates which 

 act like so many small paddles. They have as a rule 

 two long and very contractile tentacles armed with adhe- 

 sive bodies which aid in the capture of prey. Nearly all 

 of the Ctenophores are beautiful, transparent, jelly-like 

 animals, and all of the species are confined to the sea. 



The Porifera, or Sponges, were formerly regarded as 

 vegetable growths, partly no doubt on account of their 

 attachment and mode of growth, and partly because 

 they show but a slight degree of activity. The structure 

 of a sponge is best studied in one of the calcareous sponges 

 such as Grantia. The body of Grantia is cylindrical 

 with a central cavity which opens outward by a mouth, 

 or osculum. The sides are perforated by pores leading 

 to canals which open into the central cavity. Through 

 these canals a current of water is carried from the outside 

 to the central cavity, by means of the beating of flagella. 

 The minute organisms carried by these currents supply 

 the sponge with its food which is digested within the bod- 

 ies of the cells lining the canals (intracellular digestion). 

 The sponge has no organs of circulation or excretion, 

 and no nervous system or sense organs. Like Hydra, 

 the body has an inner layer, or entoderm, and an outer 

 layer, or ectoderm; but between these are other cells some 

 of which form the skeleton, or supporting tissue, of the 

 body. In the calcareous sponges this consists of spic- 

 ules of carbonate of lime. In other sponges the skeleton 

 may be composed of silica as in the beautiful glass sponges. 



