CRUSTACEA 



95 



gests, they have ten " feet." The first pair is very large and 

 armetl with large strong pincers or chelce, for defense or for 

 securing their prey. Their eyes are on movable stalks and 

 can be withdrawn under the rostrum or beak for protection. 

 The anterior thirteen segments are covered by a chitinous 

 calcareous shield called the carapace. 



The Crayfish (Fig. 68) is the best known inland example 

 of this order. The twenty segments may be discerned by 

 counting one segment to each pair of appendages, which are 

 arranged in the following order: one pair of antennules, one 

 pair of antennae, one pair of mandibles, two pairs of maxillae, 

 three pairs of maxillipeds, five pairs of legs, six pairs of swim- 



Fig. 69. — Longitudinal section through Astacus fluviatilis: C, Heart; 

 Ac, cephalic aorta; Aa, abdominal aorta; the sternal artery (Sta) is given 

 off close to its origin; Km, masticatory stomach; D, intestine; L, liver; 

 T, testis; Vd, vas deferens; Go, genital opening; G, brain; A^, ganglionic 

 cord; Sf, lateral plate of the caudal fin; o, eye stalk. (Huxley.) 



merets, or nineteen pairs of appendages and a terminal segment 

 without appendages, called the telson, which contains the 

 vent or posterior opening of the alimentary tube. 



Its locomotion on four pairs of legs may be forward, sideways, 

 or backward. Its backward locomotion by its " tail fin " 

 is probably its best and most rapid mode of locomotion. 



Digestion. — The food is seized by the cheliped and may be 

 conveyed directly to the mouth, or, after being torn into bits, 

 may be transferred to the pincers of the second and third pairs 

 of legs and from there to the mouth. The jaws move from side 

 to side instead of up and down. From the mouth the food 

 passes into the esophagus, which is very short, as the stomach 

 is in the head (Fig. 69). In the inner w^alls of the stomach 



