CUTICLE AND EPIDERMIS 201 



serve as sense organs of various kinds. From time to time the 

 cuticle is shed and a new one secreted ; this allows of growth. 

 Moulting takes place frequently while the animal is young, 

 but the old male sheds its cuticle only twice a year, and the 

 female only once. As the time for moulting draws near, a new 

 cuticle begins to form under the old one, which is loosened 

 from the epidermis, and the crayfish goes into hiding for some 

 days while the new cuticle is soft, and the animal is helpless 

 while it is hardening. The shell then splits across the back and 

 along the limbs, and the crayfish, lying on its side, draws itself 

 out of the old cuticle. 



SKELETON, MUSCLES, AND LOCOMOTION 



There is in the crayfish no continuous muscular body-wall, 

 but numerous muscles, composed of striped fibres, move the 

 various parts of its body, being attached to the inside of the pieces 

 of the armour. Thus the skeleton is external, not, like that of a 

 vertebrate, internal. Its pieces, known as sclerites, usually abut 

 upon one another across the soft jointing membranes by hard 

 knobs which serve as hinges. In the thorax ingrowths of the cuticle 

 provide a kind of false internal skeleton. This has the form of a 

 complicated scaffolding along the ventral side of the animal 

 and is known as the endophragmal skeleton. In the limbs, as in 

 those of vertebrates, opposing muscles (flexors and exten- 

 sors) bend and straighten each joint. Ingrowths of the cuticle 

 serve as tendons for them. The abdomen also is moved by two sets 

 of muscles (Figs. 136, 138). A dorsal set of extensors starts from 

 the inside of the carapace and is inserted into the terga of the 

 abdominal somites. When they contract, these muscles draw 

 forward the terga and thus straighten the abdomen. Ventrally, 

 powerful and complicated flexors connect the sterna with one 

 another and with the endophragmal skeleton (Fig. 138). The 

 flexors, when they contract, draw closer the sterna and thus 

 bend the abdomen. By this movement, spreading at the same 

 time its tail fan, the crayfish can suddenly jump backwards to 

 escape from its enemies. Its gentle forward movements are 

 carried out by the walking legs, aided by a paddhng of the 

 abdominal Hmbs. The legs of the first three pairs pull and 

 those of the last pair push, and their movements are carried 

 out in such a way that the animal is always standing upon 



