DIVISION OR TYPE VII.: SPONGES (PORIFERA). 



Animals of asymmetrical shape, aquatic, with a canal system originating 

 in numerous small pores on the surface, terminating in large oscula. 



The Fresh-Water Sponge (Spongilla). 



A. Occurrence. 



Fresh-water sponges (several species of which are distinguished by 

 naturalists) are frequently found in lakes, ponds, rivers, and canals in 

 the form of green (see p. 465, note) or grey masses of various sizes, and 

 attached to stones, piles of bridges, and other objects. 



B. Shape. 



Their shape shows considerable variation. They sometimes occur in 

 the form of crusts or lumps ; sometimes they are branched like trees and 

 antlers. In many cases they are thoroughly asymmetrical in shape. 

 This is surprising, since in their structure animals with few exceptions 

 exhibit great symmetry. Such symmetry is, in fact, a necessary 

 condition for most animals ; for a freely-moving animal could no more 

 be of asymmetrical shape than could a locomotive steam-engine (proofs). 

 On the other hand, stationary engines and sessile animals need not be 

 constructed on a symmetrical plan. 



C. Structure. 



(a) It is difficult to imagine a shapeless lump like a fresh-water 

 sponge to be a living animal, and yet such it certainly is. This we can 

 easily prove by placing a sponge which has been detached rapidly in a 

 vessel with water, and repeating our experiment with the pigment 

 granules (see crayfish and fresh - water mussel). If the granules 

 approach too close to the large apertures which we see on the surface 

 of the sponge, they are swept away, which shows that a current of 



