438 CRUSTACEA MALACOSTRACA. 



cesses, an anterior cutting point or blade, and a posterior "molar " 

 surface, separated from the anterior process by a deeper or 

 shallower notch. In the notch a row of movable spines may be 

 present, the anterior of which, larger than the others, was named 

 by Hansen the lacinia mobilis (Fig. 280). Curiously enough this 

 accessory blade is, as pointed out by Boas and Hansen, char- 

 acteristic of the mandibles of those divisions of the Malacostraca, 

 in which the young are developed in brood pouches, and absent 

 in the others (p. 454). The distal segment of the protopodite, to- 

 gether with the endopodite, constitute the 3- join ted mandibular 

 palp, which is only absent in the Cumacea, wood-lice, Atyidae, the 



zoaea larva and a 

 few other cases. 

 In the first 

 maxilla the first 

 and third seg- 

 a b. ments, according 



FIG. 280. The cutting edges of the mandibles, a of Euphausia ^ xlansen, are 

 pellucida (Euphausiidae), b of Anchialus typicus (Mysidae), j ,, _ _ j 



the latter showing the lacinia mobilis (l.m.). P r( 



wards as setose 



cutting blades. The endopodite may be a small lobe in continua- 

 tion of the axis of the limb, but in the Cumacea, Chelifera and 

 Lophogastridae it is longer and strongly reflexed, and in the 

 Leptostraca it is a long many- jointed flagellum (Fig. 284). A 

 small lobe often projects on the outer surface of the limb, 

 and may represent the exopodite. The segments of the 

 protopodite of the second maxilla are produced inwards into 

 two masticatory lobes which may, as in Astacus, be subdivided. 

 A 1- or 2-segmented endopodite is frequently present and the 

 exopodite forms a more or less fanlike plate, which in the Deca- 

 pods, where it is large and known as the scaphognathite, regulates 

 the flow of water through the respiratory chamber. Both 

 endopodite and exopodite are reduced or absent in the Cumacea, 

 Isopods and Amphipods. In the parasitic forms of the last 

 two orders the mouth parts are modified in relation to the 

 suctorial habits. 



A negative feature of the cephalic appendages is the absence 

 of the branchial epipodite or epipodites frequently found on 

 those of the thorax. 



* Not the two basal segments, as usually stated. 



