366 THE CRAYFISH CHAP. 



meres arranged in a single series, strongly calcined, 

 and with the exception of the second and third, which 

 are fused movably articulated with one another. The 

 second podomere, counting from the proximal end, bears 

 a many- jointed feeler-like organ (ex), and from the first 

 springs a thin, folded plate (ep) having a plume-like gill (g) 

 attached to it. The first two segments of the axis form 

 the protopodite (pr. I, 2), its remaining five segments 

 the endopodite (en. I 5), the base of which is toothed ; 

 and the feeler, which is directed outwards, or away 

 from the median plane, the exopodite (ex). The folded 

 plate is called the epipodite : in the natural position of 

 the parts it is directed upwards, and lies in the gill- 

 cavity between the proper wall of the thorax and the 

 gill-cover. 



The five legs (8) differ from the third maxilliped 

 in their greater size, and in having no exopodite : in the 

 fifth or last the epipodite also is absent. The first three 

 of them have undergone a curious modification, by 

 which their ends are converted into pincers or chela : 

 the fourth segment of the endopodite (sixth of the entire 

 limb, en. 4) is produced distally so as to form a claw-like 

 projection (en. 4'), against which the terminal segment 

 (en. 5) bites. The first leg is much stouter than any of 

 the others, and its chela is of immense size and forms 

 an important weapon of offence and defence. The 

 second maxilliped resembles the third, but its endopodite is 

 smaller : the first (6) has its endopodite greatly reduced, 

 the two segments of its protopodite large and leaf-like, 

 and no gill is connected with the epipodite. 



The head bears a pair of mandibles and two pairs of 

 maxillcB in relation with the mouth, and in front of that 

 aperture a pair of antennules and of antenna. The hind- 

 most appendage of the head is the second maxilla (5), a 

 leaf-like appendage, its protopodite being cut up into 



