332 THE CRAYFISH AND THE ARTHROPODA 



"antagonistic" muscle attached, like those of the abdomen, 

 to the inside of the skeleton on either side of a movable joint 

 (Fig. 160). Such a relationship of muscles to skeleton is charac- 

 teristic of the Arthropoda and may be compared with that seen 

 in an animal with an endoskeleton (c/. Fig. 22, p. 42). Since the 

 locust will be studied intensively as another representative of the 



Fui. 160. — First pereiopod of lobster cut open to show relation of muscles to 



skeleton. 



Arthropoda, the internal structure and special activities of the 

 crayfish will not be further described. The eggs of the crayfish 

 are attached to the swimmerets of the female at the time of laying 

 and remain in this position until the young are hatched (Fig. 155). 

 The young remain attached to these appendages by means of their 

 chela; until a stage that resembles the adult in miniature (Fig. 161). 



Other Crustacea 



Classification. — In an older classification, based upon less knowl- 

 edge than is now available, the Class Crustacea was divided into 

 only two sub-classes, the Entomostraca and the Malacostraca. Such 

 an arrangement is still useful, if we do not need to burden our- 

 selves with too much terminology, because it separates the more 

 highly developed types, like the crayfish, from the crustaceans of 

 simpler organization. It is true that many of the Entomostraca, 

 like the water-flea, Daphnia (Fig. 162), are greatly specialized; 

 but some of them, like the " fairy shrimp," Branchineda (Fig. 162) 

 are the simplest of all Crustacea in their metamerism and in the 

 arrangement of their appendages. Hence, one often speaks of the 

 Entomostraca as the " lower," and the Malacostraca as the 

 * ' higher ' ' Crustacea. 



The Malacostraca. — To these " higher " Crustacea belong 

 the forms that are popularly known as the sow-bugs, scuds, 

 shrimps, prawns, crayfishes, lobsters, and crabs (Fig. 163). 

 Although the great majority are marine, many occur in fresh water; 



