CCELENTERA TA. 143 



between the Ccelenterata and most of the other 

 Metazoaj that it has been proposed to divide them 

 from these, and place them in a separate and lower 

 grade. There are, however, indications of the third 

 layer even in Hydra, and in others of the Ccelenterata 

 it is much more conspicuous ; but it differs greatly in 

 character from the third layer of higher animals, and 

 is therefore usually referred to under the special name 

 of mesogloea;.'^ In Hydra it is spoken of as the 

 middle lamina, or as the supporting lamella.^ 



The Ccelenterata include the Pokieeea or Sponges, 

 a group which differs from other animals in having a 

 branched digestive cavity (enteron), and the Cnidaeia, 

 or nettle-animals, so called from their power of sting- 

 ing their prey. 



The PoEiFEEA have been, one of the puzzles of 

 Zoology, and it is only quite lately that it has been 

 possible to compare the structure of the sponge with 

 that of other animals. A sponge consists of a jelly- 

 like substance strengthened by a skeleton of hard 

 matter, which, in the dead, and dry state, forms what 

 is known commercially as sponge. In the live state, 

 there is a circulation of water through the canals which 

 permeate the substance of the sponge. 



The circulation in the canals is maintained by the 

 cilia of the cells which line them. This lining layer 



'■ A name given on account of its texture (Gk. yKoia, glue 

 or anything sticky). 

 ^ Diminutive of Lat. lamina, a very thin plate. 



