234 ARTHROPODS. 



sists of a two-jointed basal piece (protopodite), and two jointed 

 branches rising from this — an internal endopodiie and an 

 external exopodite ; but in many, the outer branch disappears. 

 We can fancy the origin of this form of limb from the similarly 

 double " parapodium " of a Polychste. The hard chitinous 

 cuticle of the Arthropods makes joints possible and necessary. 

 In regard to the following list of appendages, be it noted that 

 the eye-stalks are no longer included in the series since their 

 development is not that of the limbs, and, moreover, that 

 though the two pairs of antennae lie far in front of the mouth, 

 it is likely that they have been shunted forward from a post- 

 oral position. With many of the thoracic appendages, gills, 

 plate-like epipodites, and setae are associated. 



An attempt should be made 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 

 last joint of the exopodite bends in such a way that the 

 friction is reduced. From the smallness of the exopodite in 

 the third maxillipede, one may argue by analogy that in the 

 succeeding thoracic appendages with only one fork the 

 exopodite has disappeared. We may picture the gradual 

 development of the claw by the irritation produced by the 

 last joint biting against the second last. It is very likely 

 that some of the crowded mouth parts, e.g., the first maxillae, 

 are almost functionless. The hard toothed knob which forms 

 the greater part of the mandible is obviously well adapted to 

 its crushing work. With the great length of the endopodite 

 in the antennae, the reduced state of the exopodite may be 

 connected. 



In connection with the skeleton, the student should also 

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

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

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

 the flaps of the head-and-thorax shield ; that each posterior 

 segment consists of a dorsal arch (terguni), 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 ; and that the hindmost piece {telson) on which 

 the food-canal ends ventrally is regarded by some as a 



