352 THE CRAYFISH chap. 



main portion of the limb is formed of seven podomeres 

 arranged in a single series, strongly calcified, and — with the 

 exception of the second and third, which are fused — move- 

 ably 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), 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 chelce : the fourth segment of the 

 endopodite (sixth of the entire limb, en. 4) is produced dis- 

 tally so as to form a claw-like projection {en. 4'), against 

 which the terminal segments {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 is considerably smaller : ihs. first (6) has its endopodite 

 greatly reduced, the two segments of its protopodite large 

 and le>if-like, and no gill is connected with the epipodite. 



The head bears a pair of mandibles and two pairs of 

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

 aperture a pair of antennules and. one of antenna. The 

 hindmost appendage of the head is the second maxilla (5), 

 a leaf-like [appendage, its protopodite being cut up into 



