8 THE PHYSICAL KINSHIP 



and sponges^ secrete plant-like frames, upon and 

 among the branches of which the organisms 

 reside ; and still others, as the clams, crustaceans, 

 and insects, have skeletons consisting of a shell 

 or sheath on the outside of, and more or less 

 surrounding, the softer substances of the body. 

 The limbs of insects are tiny tubes on the inside 

 of which are the miniature muscles with which 

 they perform their marvels of locomotion. The 

 skeleton of vertebrates, consisting of levers, beams, 

 columns, and arches, all skilfully joined together 

 and sunk deep within the muscular tissue, forms a 

 conspicuous contrast to the rudimentary frames of 

 other animals. The vertebrate skeleton consists 

 of a hollow axis, divided into segments and ex- 

 tending along the dorsal region of the body, from 

 the ventral side of which articulate, by means of 

 awkwardly-constructed girdles, an anterior and a 

 posterior pair of limbs. This dorsal axis ends in 

 front in a peculiar bulbous arrangement called the 

 head, which contains, among other valuables, the 

 brain and buccal cavern. The thoracic segments 

 of the backbone send off pairs of flat bones, which, 

 arching ventrally, form the chest for the protection 

 of the heart and other vitals. The limbs (except 

 in fishes) consist each of a single long bone, 

 succeeded by two long bones, followed by two 

 transverse rows of short, irregular wrist or ankle 

 bones, ending normally in five branching series of 

 bones called digits. This is essentially the skeleton 

 of all fishes, amphibians, reptiles, birds, and 

 mammals. In short, it is the universal vertebrate 



