44 BIOLOGY 



whip-like outgrowths, is exhibited by the spermatozoa 

 only. 



It is only natural that the animal with its more active 

 and more hazardous life should have a greater variety 

 of, and more specialised forms of, locomotory organs. 

 There are, to name a few types only the limbs of 

 mammals, the wings of birds, the jointed limbs of crabs 

 and insects, the tube feet of the sea-urchin, and the 

 enormous muscular foot of the molluscs. 



This leads naturally to the question of the skeleton 

 or hard parts of the organism. 



What are the functions of the skeleton ? It is mani- 

 festly clear that it protects the soft parts. The skull, 

 for example, protects the brain ; the shell of the turtle, 

 the crab and the mussel, the hard outer-covering of the 

 beetle, protects the whole body ; and similar functions 

 are performed by the hair of mammals, the skin of the 

 reptiles and of the rhinoceros. In plants in like manner 

 cork and corky cuticles are protective in function, so 

 also are thorns, spines, resin, &c. Another function of 

 the skeleton is to give rigidity, well exhibited in the 

 limbs, while their mobility is not interfered with, owing 

 to the development of joints, but this rigidity is also 

 seen in the venation of a leaf, where the arrangement 

 of the veins which are skeletal structures keep the leaf 

 from crumpling up and also from being destroyed, for 

 example, by high winds. 



In animals the skeleton also serves another purpose, 

 in that it gives points of attachment for the muscles of 

 the body, and thus enables the parts to move singly or 

 in concert. In plants the skeleton combines still another 

 function with those of protection and rigidity, namely, 



