166 HOMOLOGY 



form and appearance, are serially homologous with one another. 

 The Crustacea therefore give excellent subject matter for the 

 study of homology. 



I. THE AMERICAN LOBSTER, HOMARUS AMERICANUS 



HABITS, MODE OF LIFE. Lobsters live in comparatively 

 shallow waters along the Atlantic coast from Labrador to Dela- 

 ware, in depths of from one to 100 fathoms. They are preda- 

 tory, but usually capture their prey by stealth, while hidden in 

 weeds on the sea bottom. They are also well-known scavengers, 

 and will quickly discover and devour dead fish to which they are 

 attracted through an acute sense of smell. Ungainly on land, 

 their movements in water are graceful, where they may run 

 about with agility or shoot backward with surprising speed. 

 When enemies are about they are pugnacious, but at the same 

 time wary and resourceful, and are well able to defend them- 

 selves. A closely related species is the European lobster 

 (Homarus gammarus), while somewhat similar forms are the 

 langouste of the French coast, and the so-called Norwegian 

 lobster (Nephrops norvegicus) . 



GENERAL STRUCTURE AND SYSTEMS or ORGANS. Like the 

 earthworm, the lobster and all of its allies are metameric ani- 

 mals. The somites or metameres may easily be seen in the 

 abdomen, where they are separate, but in the anterior region 

 they are fused together, those of the head (cephalon) fusing 

 with those of the thorax to form the main part of the animal, 

 termed the cephalothorax (Fig. 67). All parts of the body are 

 covered by a firm lifeless cuticle of chitin which, on the back 

 (dorsum) and on the side (tergum), is impregnated with calcium 

 salts, until quite solid. In some species this covering becomes 

 almost rock-like in its solidity, containing much pure limestone. 

 The chitin and lime are secreted by cells of the skin, which is 

 drawn down over the sides of the cephalothorax in two great 

 folds like the front flaps of a coat; the two flaps thus cover 

 and protect two branchial chambers on the two sides of the cepha- 

 lothorax, where the gills lie, and are called the branchiostegites 



