n6 ECONOMIC ZOOLOGY AND ENTOMOLOGY 



barnacle, is attached to floating objects by means of a flexi- 

 ble stalk which may be very short or may attain a length of 

 nearly a foot. In warm seas the bottoms of ships are often 

 so covered with these barnacles that their progress is 

 seriously impeded. 



The very degenerate parasitic Sacculina also belongs to this 

 order. The young sacculina swims about freely, but soon 

 attaches itself to the body of a crab. After undergoing a 

 series of changes, one stage of which is passed within the body 

 of the crab, it finally becomes little more than an ovoid sack 

 closely applied to the host, and sending off many root-like 

 filaments which extend to all of the tissues of the crab from 

 which it derives its nourishment. 



Sub-class Malacostraca. This group includes the more 

 highly organized Crustacea. Most of them are of considerable 

 size, and the appendages show much differentiation. There 

 are several orders, some including mostly fresh water, others 

 mostly marine forms, but the members of only two of these 

 orders are of any particular economic interest or importance 

 except as they furnish food for fishes and other smaller animals. 



The order Decapoda, to which the crayfish belongs, is the 

 largest and most important order belonging to the class. The 

 order is divided into two groups, or sub-orders, the Macrura 

 and the Brachyura. In the Brachyura the abdomen is usually 

 much reduced in size and folded underneath the thorax. 

 The Macrura includes the free-swimming shrimps and prawns, 

 the crawling lobsters and crayfish, and the hermit crabs, sand 

 bugs and others. 



Lobsters. As a source of food for man the lobsters rank 

 first among the Crustaceans. They are found along rocky 

 shores on both sides of the Atlantic ocean. In structure and 

 habits they are much like the crayfish, but they attain a much 

 greater size, some individuals reaching a length of twenty inches 

 and a weight of twenty-five pounds. At rare intervals even 

 larger specimens are found, but these are to be regarded as 

 giants. The lobsters seen in the market usually measure ten 

 to twelve inches and weigh from one and one-half to two and 

 one-half pounds. Many states do not allow lobsters to be 



