216 



ELEMENTS OF SCIENCE 



two layers, and contain silicious, calcareous, or horny 

 skeletons. They have also a number of inhalent and 

 exhalent apertures. 



The zoophytes, or plant-like animals (which include 



FIG. 42. 



Hydra viridis. 



So-called foot. . t 1 Tentacles not fully grown. 



h Mouth. * Prey seized. 



t One of the tentacles. 



(After Howes.) 



the coral-forming polyps, which live aggregated toge- 

 ther), the jelly fishes, sea anemones, &c., succeed the 

 sponges. The simplest form of the group is the Hydra, 

 which consists of a sack with a single aperture sur- 



