162 HOMOLOGY 



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 

 Delaware, in depths of from one to 100 fathoms. They are 

 predatory, 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. Un- 

 gainly 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 OF ORGANS. Like 

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

 animals. 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) are impregnated with calcium salts 

 until quite solid. In some species this covering becomes 

 almost rock-like in its solidity containing much pure lime stone. 

 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 cephalo- 

 thorax where the gills lie and are called the branchiostegites or 

 gill protectors. On the under side of the body, especially in 

 the abdomen, the chitin is thin and transparent with heavier 



