THE SPONGES 23 



between three and four miles from the surface. Unlike 

 the majority of animals, all members of this group are 

 securely fastened to some foreign object, such as rocks, the 

 supports of wharves, or with one extremity embedded in 

 the sand. As we have seen, the young enjoy a free-swim- 

 ming existence and are swept far and wide by means of 

 tidal currents, but sooner or later these migrations are 

 terminated ?n some suitable locality, where the sponge 

 passes the remainder of its existence. During this time 

 some species may never exceed the size of a mustard-seed, 

 while others attain a diameter of three feet, or even more. 

 Sponges also vary exceedingly in shape, some having the 

 form of thin encrusting sheets, others being globular, tubu- 

 lar, cuplike, or highly branched (Fig. 12). 



25. The influence of their surroundings. In by far the 

 larger number of cases an animal possesses the bodily form 

 of the parent. External agencies may modify this to some 

 extent, but usually only to a limited degree. A squirrel, 

 for example, resembling its parent, may grow to a relatively 

 large or stunted size according to the food supply, and it 

 may become strong or weak according to the amount of 

 exercise, and various other changes may result owing to 

 outside causes ; but as a result of these influences the 

 animal is rarely so modified that one is unable to distinguish 

 the species. Many of the sponges, however, are exceptions 

 to this general rule. If, for example, some of the young 

 of a certain parent develop in quiet water or in an un- 

 favorable locality, they will usually be low, flat, and un- 

 branched ; while the others, growing in swiftly running 

 waterways, develop into tall, comparatively delicate and 

 highly branched individuals. Under such circumstances 

 not only does the external form become modified, but 

 the internal organization may undergo profound change. 

 The entire organism is plastic and readily molded by 

 the influence of its surroundings, and the consequent 

 lack of definite characters often renders it impossible 



