XII] ASTACUS 185 



endopodite is a short unjointed filament whilst the exopodite is 

 a broad jaw -like plate called the scaphognathite. 



The first maxilla ormaxillulaisa still slenderer limb than the 

 second maxilla the exopodite is absent and the endopodite represented 

 by a tiny filament, whilst from the basal joints two gnathobases are 

 developed. The foremost of the three jaws is termed the mandible. 

 It is much stronger and thicker than any of the jaws described 

 except the third maxillipede, though in general form it recalls the 

 first maxilla. There is one strong toothed gnathobase which has 

 two edges, an outer cutting or " incisor " edge and an inner crushing 

 or "molar edge/' the exopodite is absent and the endopodite is a two- 

 jointed appendage much smaller than the gnathobase, which is 

 termed the palp. 



We can now form to ourselves some idea of the process of 

 mastication in the crayfish. The flesh of the prey is torn into 

 fragments by the chelae; these are then passed to the third maxilli- 

 pede and then gradually worked forward through the whole series 

 of jaws, becoming broken into finer and finer fragments before enter- 

 ing the mouth. The delicate maxillae appear to act like combs, 

 removing the softer particles from the more resistant ones these 

 last being finally ground up by the stronger mandibles. 



In front of the mouth there are to be found two pairs of 

 appendages which are modified to act as feelers, and by means of 

 these the crayfish explores the neighbouring water. These are the first 

 and second pairs of antennae, the first pair being sometimes termed 

 antennules (an ', Fig. 70) whilst the phrase antenna without quali- 

 fication denotes the second pair (an", Fig. 70). These appendages are 

 proved by a study of development to have originally belonged to the 

 region behind the mouth and to have been gradually shifted forwards 

 during the course of growth. It is characteristic of Crustacea as 

 distinguished from the other Arthropoda that they have two pairs 

 of appendages in front of the mouth acting as feelers. The second 

 or large antenna commences by a protopodite consisting of the 

 usual two joints which are very short and broad. On the under 

 surface of the coxopodite is a rather conspicuous knob of cuticle 

 the summit of which is pierced by a hole. This is the opening 

 of the kidney (g.g, Figs. 70 and 71). The exopodite is represented 

 by a flat oblong scale the squarne which is freely movable on 

 the protopodite and which serves as a floor to the eye-socket. The 

 endopodite is represented by a long filament two thirds the length 

 of the body, the basal joints of which are stout and fairly long but 

 the distal joints are disc shaped, so that the whole is exceedingly 



