MUSCULAR SYSTEM. 257 



We can fancy how the Crustacean form of limb might arise from the 

 biramose parapodium of a Polychsete. The hard chitinous cuticle of 

 the Arthropod makes joints possible and necessary. In regard to the 

 foregoing list it should be noted that the eye stalks are no longer 

 included in the series since their development is not like that of the 

 limbs, and, moreover, that though the two pairs of antennae lie far in 

 front of the mouth, it is possible that they were originally post-oral. 

 With many of the thoracic appendages, gills, plate-like epipodites, and 

 setae are associated. 



It is interesting to connect the structure of the appendages with their 

 functions. Thus it may be seen that the great paddles are fully spread 

 when the crayfish drives itself backwards with a stroke of its tail, while 

 in straightening again the paddles are drawn inwards, and the outer 

 joint of the exopodite bends in such a way that the friction is reduced. 



It is likely that some of the crowded mouth parts, e.g., the first 

 maxilke, are almost functionless. The hard toothed knob which forms 

 the greater part of the mandible is obviously well adapted to its 

 crushing work. 



In connection with the skeleton, the student should also 

 notice the beak (rostrum} projecting between the eyes ; the 

 triangular area (epistoma) in front of the mouth, and the 

 slight upper and lower lips ; how the gills are protected by 

 lateral flaps of the body wall ; that each posterior segment 

 consists of a dorsal arch (tergum\ side flaps (pleura), a 

 ventral bar (sternum], while the little piece between the 

 pleura and the socket of the limb is dignified by the name 

 of epimeron. The hindmost piece (telson) on which the 

 food canal ends ventrally is regarded by some as a distinct 

 segment, by others as an unpaired appendage. The most 

 difficult fact to understand clearly, is that the cuticle of 

 certain mouth parts (e.g., the mandibles), and of the ventral 

 region of the thorax, is folded inwards, forming chitinous 

 " tendons " or insertions for muscles, protecting the ventral 

 nerve cord and venous blood sinus, and above all, con- 

 stituting the complex, apparently, but not really, internal, 

 " endophragmal " skeleton of the thorax. 



Muscular System. 



The muscles are white bundles of fibres. On minute 

 examination these show clearly that transverse striping 

 which is always well-marked in rapidly contracting ele- 

 ments. They are inserted on the inner surface of the 

 cuticle, or on its internal foldings (apodemata). The most 

 important sets are (i) the dorsal extensors or straighteners 



17 



