88 PRINCIPLES OF ANIMAL BIOLOGY 



the mussels they are too soft and deHcate to do so unaided, and it is the 

 shell which holds them in place. 



The importance of the skeleton is closely related to size of body and 

 the place where the animal lives. A large animal may exist in the sea 

 and, because the body is of about the same density as the surrounding 

 water, be buoyed up in such a way as to allow its parts to function. 

 Cuttlefishes, for example, lead active lives in marine waters but washed 

 up on shore are helpless and shapeless. On land, however, even moder- 

 ate-sized mammals, because the medium around them, the air, is so much 

 lighter than themselves, would be unal^le to maintain the physical rela- 

 tions of their parts to one another sufficiently to enable them to function 

 if they were made of mere protoplasm. Some form of mechanical sup- 

 port other than a skeleton might have been evolved; but large size with- 

 out such support, along with physiologies of the general sort exhibited 

 by modern land animals, would have been out of the question. 



Skeletons and Protection. — Nearly every skeleton may be regarded 

 as a source of protection, though often there is little definite information 

 to show what injuries might result in the absence of the skeleton. Those 

 sponges which have a skeleton of limy spicules generally bristle all over 

 with long shafts projecting from the surface cells (Fig. 74). How much 

 they are thus protected from predatory animals can only be 

 conjectured. In some marine animals known as hydroids, 

 having the general structure of Hydra but existing in 

 branching colonies, there is a horny tubular-sheath covering 

 the various branches and main stem of the colony. This 

 skeleton enables the hydroids to stand out more or less 

 firmly instead of being lashed against other objects by the 

 Fig. 74.— waves. In insects, crayfishes, spiders, and their allies 

 simp e i\^Qy.Q jg Q^ skeleton of a horny substance known as chitin 



sponge. -^ 



{From Heo- which covers the entire l^ody on the outside. This does 

 ^Zooloav " The ^^^ protect them from predatory animals, since members 

 Macmillan of this group, particularly the insects, are abundantly 

 ompany.) eaten by other animals; but it must serve to ward off 

 mechanical injuries of other kinds. The limy wall, or test, of sea urchins 

 and the shells of clams are presumably likewise protective structures. In 

 the vertebrate animals some of the most delicate and vital organs are 

 within bony cases — the brain within the skull, the spinal cord in a canal 

 running through the backbone, the heart within the framework of the 

 chest, and such sense organs as the cars and eyes either imbedded in 

 solid bone or set in among projecting ridges or other prominences. 



Skeletons which serve only the functions of supi)ort and protection 

 may often be rigid one-piece structures. Some of the protozoa have a 

 solid limy shell sm-rounding the whole cell, and corals rest in limy cups 



