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 in 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 
3 
