238 



COMPARISON BETWEEN ANIMALS AND PLANTS. 



Fig. 165. 



stances ; it consists of a fibrous mass, containing a jelly-like 

 substance, wliich, when touched, discovers a slight sensation, 

 the only sign of life manifested by it. There are many species 

 of sponge ; those most valued in the arts are found in the Med- 

 iterranean Sea, and Indian Ocean. Some grow upon rocks, and 

 are found covering the interior of submarine caves. The Spo7i 

 gia parasitica is seen growing upon the back and legs of a 

 species of crab ; sometimes as many as forty individual sponges 

 extend themselves over the crab impeding the motion of its 

 joints, spreading like a cloak over its back, or forming for its 

 head grotesque and towering ornaments, from which the poor 

 crab vainly attempts to disencumber itself. Some species of 

 the sponge grow to a very large size ; one has been found in 

 the East Indies in the form of a cup capable of containing ten 

 gallons of water. The fibrous part of the sponge is the skele- 

 ton of the animal; the large apertures (see Fig. 158, ^) serve 

 to carry out fluids from within ; while the water by wh'xh the 

 animal is nourished, is imbibed by minute pores : this contin- 

 ual circulation of water is one of the most important functions 

 of the living sponge. These animals resemble plants in their 

 r)%anner of producing others', they form a species of germ, like 

 the bud growing upon the stalk ; this falls from the stem and 

 b'ecomes a perfect animal. If a part of one of these animals 

 be separated from the rest, it will, itself, be as perfect a living 

 animal as was the whole before. A polypus can be divided 

 into as many animals as it contains atoms ; some of this order 

 are very properly called hydras (many-headed). Besides these 

 there is another order of animal substances, infusoria^ which 

 appear like a homogeneous mass, having no appearance of any 

 limbs whatever ; these are either angular, oval, or globular. 



Manner in which these animals are reproduced— Recapitulation. 



