WORMS TO VERTEBRATES 67 



oxygen at each circuit. But in the insect the blood 

 conveys only the dissolved solid nutriment, the food ; 

 hence a comparatively irregular circulation answers all 

 purposes. 



The skeleton is a thickening of the horny cuticle of 

 the annelid on the surface of each segment. The horny 

 cylinder surrounding each segment is composed of 

 several pieces, and on the abdomen these are united by 

 flexible, infolded membranes. This allows the increase 

 in the size of the segment corresponding to the vary- 

 ing size of the digestive and reproductive systems. In 

 this part of the body the skeletal ring of each segment 

 is joined to that of the segments before and behind it 

 in the same manner. But in other parts of the body 

 we shall find the skeletal pieces of each segment and 

 the rings of successive segments fused in one plate of 

 mail. The legs are the parapodia of annelids carried 

 to a vastly higher development. They are slender and 

 jointed, and yet often very powerful. A large portion 

 of the muscular system of the body is attached to these 

 appendages. 



But the insect has also jaws. The annelid had teeth 

 or claws attached to the proboscis. But true jaws are 

 something quite different. They always develop by 

 modifying some other organ. In the insect they are 

 modified legs. This is shown first by their embryonic 

 development. But the king- or horseshoe-crab has still 

 no true jaws, but uses the upper joints of its legs for 

 chewing. There are primitively three pairs of jaws of 

 various forms for the different kinds of food of different 

 species or higher groups. But some of them may dis- 

 appear and the others be greatly modified into awls 

 for piercing, or a tube for sucking honey. Into the 



