142 THE CRAYFISH. 



which is subservient to respiration, the scapho- 

 gnathite. 



i. The protopodite consists of two segments, eoxopo- 

 dite and basipodite, each of which is expanded 

 on its inner side into a bilobed lamella. 



ii. The endopodite is small, pointed, and unjointed. 



iii. The scaphognathite or exopodite is an elongated 

 plate attached by the middle of its inner edge 

 to the outer side of the basipodite. It serves 

 to keep a current of water flowing through 

 the gill-chamber (p. 146). 



IV. The first maxilla is small. The two segments of 

 the protopodite extend inwards as thin curved 

 lamellse, and the endopodite is a small plate to 

 their outer side. There is no exopodite. 



III. The mandible consists of a very stout undivided 



protopodite, forming the powerful jaw at the side 



of the mouth ; and an endopodite, which forms a 



small three-jointed palp, lying in a groove on the 



front of the protopodite. 



Before removing the mandible, note the position of the two 



pivots about which it turns, and the points of insertion of the 



tendons of its abductor and adductor. 



Immediately in front of the first maxilla and 

 behind the mandible is a small unjointed plate, leaf- 

 like in form. It is not regarded as a true appendage, 

 but as an outgrowth of the metastoma, which forms 

 the posterior and lateral borders of the mouth. 



II. The antenna consists of protopodite, exopodite, and 

 endopodite, the last of which forms the long 

 ' feeler.' 



i. The protopodite consists of two segments ; the first, 

 or coxopodite, being very broad, and projecting 

 only very slightly from the surface of the head ; 

 on its ventral surface is a tubercle perforated 



