180 ARTHROPOD A [CH. 



Crustacea, such as the Shrimps, and by means of them the animal 

 is enabled to swim forwards. The crayfish is too bulky to be 

 completely supported by the action of these appendages but when 

 the crayfish . creeps forwards by means of its other appendages, 

 these by their vigorous motion support the abdomen and prevent 

 it from resting on the ground, and aid the whole forward movement. 



The appendages borne by the first two segments of the abdomen 

 are obviously constructed on the same plan as those borne by the 

 other segments but they have undergone great modification. From 

 the fact that they differ greatly in the two sexes we should be led 

 to suspect what proves to be the case, that this modification has 

 taken place in consequence of their being used in connection with 

 the sexual function. As is often the case with sexual appendages 

 they vary much in shape from species to species and hence are of 

 great value in classifying crayfish. In the male of Cambarus virilis 

 the endopodite of the appendage belonging to. the second segment 

 is in its basal portion transformed into a rigid unjointed rod, whilst 

 the endopodite of the appendage belonging 'to the first segment is 

 transformed throughout its whole extent except the extreme tip into 

 a grooved rod, whilst the exopodite is completely absent. When 

 the appendages of the two segments are pressed together a tube 

 is formed by the juxtaposition of the endopodites on each side, and 

 through this tube the germ cells of the male are conveyed to the 

 female. In the female the appendages of the second segment are 

 like those of the third, fourth and fifth, whilst the appendages of the 

 first segment are reduced to tiny vestiges. 



Turning now to the carapace which covers the cephalothorax 

 we find that it is divided into an anterior and a posterior portion by 

 the cervical groove (cv.g, Fig. 70), a well marked transverse 

 groove which crosses it. The portion of the animal in front of this 

 groove is termed the head, whilst the part behind is called the 

 thorax. The thorax is marked by two longitudinal grooves which, 

 starting from the cervical groove run backwards to the hinder edge 

 of the carapace. The two grooves are termed branch iocardiac 

 grooves (br.c, Fig. 70) and the lateral areas marked off by them are 

 known as the branchiostegites or gill-covers (Gr. /Jpay^ia gills, 

 oreyav to cover) (be, Fig. 70) and do not really form the lateral walls 

 of the body but form two flaps which project at the sides and 

 between which and the body the gills are lodged. 



If we examine the under-side of the thorax we observe that it 

 bears a number of long jointed appendages, between the bases of 



