THE LOWEST GROUP OF ANIMALS. 45 



is most probable that nervous matter, in the form 

 of a subtle fluid, or of atoms hitherto beyond detec- 

 tion, may be blended with the gelatine. True 

 blood vessels do not exist ; yet in some groups 

 canals are excavated in the substance of the gelatine, 

 through which absorbed fluids circulate, and are 

 carried to a central cavity. This apparatus fulfils 

 at once the double office of aerating the system, (for 

 oxygen, as we have seen, is indispensable to animal 

 life,) and of supplying it with nutriment ; which it 

 cannot be doubted is absorbed and assimilated. In 

 some of the simplest animals, indeed, a more perfect 

 set of alimentary and aerating organs is present, and 

 their use cannot be mistaken. The power of loco- 

 motion enjoyed by these beings differs exceedingly ; 

 and many, fixed like plants, live and die on one 

 spot. None have true limbs, but many have 

 tentacles, arms, or feelers, by means of which they 

 secure their prey. 



To give an example : all are familiar with the 

 lightness, softness, and elasticity of sponge. It 

 exhibits generally the same structure, having 

 numerous pores and circular holes, which lead into 

 canals permeating the whole substance. It imbibes 

 with great facility a large quantity of water, or 

 indeed any fluid, which is held in the meshes of its 

 rude and singular network, until, being forced out 

 by pressure, it acquires its former bulk. 



The name of sponge is derived from the Greek 

 word to squeeze, and shows that it was early noticed 



