v CRUSTACEA MUSCULATURE 335 



The mechanism for the movement of the appendages is in principle the same as 

 that of the body. It is, however, evident that if the hinges of the joints of an 

 appendage were arranged at equal distances along two parallel straight lines the row 

 of joints of the extremity could only bend in one plane, as is indeed the case in the 

 row of body segments. But the two hinges of the consecutive joints are in reality so 

 placed as to make free movement possible. The exoskeleton of the body segments is 

 related to the exoskeleton of the basal joints of the appendages which it carries just 

 as a large joint of a limb is to a smaller. The muscles which on the one hand are 

 inserted in the exoskeleton of the first joints of the extremities, by preference attach 

 themselves on the other to the tergum of the corresponding segment. 



The mechanism of the forceps (chelse) of the Cray -fish (Fig. 233) is as follows. 

 The forceps is formed by the two terminal joints of the ch elate foot. The last 

 joint but one is produced into a pointed process (zf). The last joint (eg) articu- 

 lates with it in the ordinary way by means of two opposite hinges. 



Round the hinges the two joints are connected by means of a thin and flexible 

 interarticular membrane. Two muscles serve in the way shown in the illustration 



FIG. 233. Forceps of the large chelate foot of the Cray-fish, diagrammatic. A, closed. B, 

 Open, eg, Terminal joint ; vg, last joint but one with the pincer process zf; a, hinge on which the 

 terminal joint moves ; gh, interarticular membrane ; o, opening muscle (abductor) ; s, closing 

 muscle (adductor). 



for moving the terminal joint. By the contraction of the smaller the movable joint 

 describes an arc away from the fixed process (Fig. 233, ), opening the forceps. If 

 the much stronger muscle which lies on the other side of the hinge contracts the 

 chela closes (A). 



The muscles of Crustaceans are often attached to the exoskeleton 

 by means of sinewy and even chitinous terminal pieces. In the latter 

 case we can speak of an endoskeleton. Both arrangements serve for 

 the increase of the surface of attachment of the muscles. 



In those Entomostraca in which the strongly -developed shell -fold 

 surrounds the whole body as a bivalve shell (Ostmcoda, Estheridce), a 

 strong shell-musele connecting the two valves transversely serves for 

 closing the shell. We find such a shell-muscle among the Malacostraca, 

 also in the Leptostraca (Nebalia). 



As in all Arthropoda, the body musculature is transversely 

 striated. 



