2 THE MECHANISM OF LIFE 



soft mass in the cavity of tlie head. This is the brain, and 

 further examination shows that it is connected, by means of nerves, 

 with the great sense organs of the head — that is, the eyes, ears, 

 nose and tongue — but also with a similar mass of nervous tissue 

 lying inside a tubular cavity in the backbone. This is the spinal 

 cord, or marrow. Looking at it more closely, he finds that the 

 spinal cord gives origin to nerves which can be traced out into 

 the muscles of the trunk and limbs, and also into the skin. 



In short, he finds that the animal body is a structure: a series 

 of parts arranged in a definite way. 



Structural Differences. — Now, extending his study to other 

 examples of the animal kingdom, he finds other structures 

 which are analogous, or similar in a general way, to that which 

 he has already examined. There are, however, very notable 

 differences. The fore and hind limbs of the mammal are replaced 

 by the wings and legs of the bird, or by the little side fins of the 

 fish. He will find that all backboned animals possess these 

 two pairs of limbs in some form or other, but that they may 

 differ remarkably in structure. Examining the animals below 

 the vertebrates, he will find still greater differences: thus there 

 are about twenty pairs of limbs (or appendages) in a crab, 

 shrimp, or lobster, but many more in a centipede, and such a 

 creature as a starfish has apparently no limbs at all (though one 

 speaks about its radial " arms "), but beneath the body he will 

 find some thousands of mobile parts which serve for locomotion, 

 and are known as the " tube-feet." The viscera also vary very 

 strikingly. Thus there are distensible lungs in the quadrupeds, 

 but these are relatively compact and inelastic in the birds, and 

 attached to them are a number of long " air- sacs." There are 

 no lungs at all in the fishes, but he will find gills there, organs 

 which are apparently wanting in the warm-blooded animals. 

 The heart, again, differs greatly in the various kinds of animals; 

 thus it has four chambers in the mammals and birds, three 

 in the frog and some other amphibians, but only two in the 

 fishes. The eyes are large and well- developed in all backboned 

 animals, but quite different in structure in insects and shellfish, 

 like the crab and lobster; different, again, in the cuttlefish, 

 rudimentary in the snail and in most worms, and wanting 

 altogether in such creatures as the oyster, mussel and cockle. 



Thus the student of comparative anatomy finds very great 

 differences of structure in the various groups of animals: differ- 



