THE CRAYFISH 117 



considering them homologous with the sessile eyes of Apus, and 

 therefore as organs belonging to an original prostomial region 

 devoid of limbs. Omitting the eyes, we can count nineteen pairs 

 of appendages, of which two pairs, the first and second antennae, 

 lie in front of the mouth. Three pairs lie close round the mouth 

 and are modified as masticatory organs ; they are the mandibles 

 and first and second maxillae. Three pairs succeeding these are 

 turned forwards, and more or less modified to form foot-jaws 

 viz. the first, second, and third maxillipeds. The next pair 

 forms the large pincers or chelae. The next four pairs are the 

 walking legs or pareiopods, and on each of the six segments 

 of the abdomen is a pair of swimming legs or pleopods, the first 

 five pairs being small, the last pair expanded to form broad 

 swimming plates lying alongside the telson. The first five pairs 

 of appendages we may regard as cephalic appendages, and it 

 should be observed that they correspond with the cephalic 

 appendages of Apus. The maxillipeds, chelae, and pareiopods 

 are thoracic appendages, and the remainder are abdominal. 



Taking the right limb of the third abdominal segment, the 

 sixteenth of the wholeseries, as an example, we find (fig. 26, XVI) 

 that it consists of a two-jointed basal stalk bearing an inner and 

 an outer branch at its extremity. The basal stalk is known as 

 the protopodite ; its very short proximal joint is the coxopodite, 

 its long and cylindrical distal joint is the basipodite. Of the 

 two distal branches the inner is known as the endopodite, the 

 outer as the exopodite. They resemble one another in all 

 essential points, but the exopodite is rather shorter than the 

 endopodite, and is articulated rather lower down than its fellow 

 to the inner margin of the basipodite. Each branch consists of 

 a proximal flattened piece bearing at its distal end a flagelliform 

 appendage imperfectly divided by transverse constrictions into 

 ten or eleven joints bearing a number of plumose setae on their 

 outer and inner margins. The fourth and fifth pairs of ab- 

 dominal limbs are practically identical with the third, but the 

 last pair, the nineteenth of the whole series of appendages, is 

 modified. The basal stalk is short and broad, and is not 

 divided into a coxopodite and basipodite. The exopodite and 

 endopodite are expanded and flattened, forming broad swim- 

 ming plates fringed at their margins with plumose setae. The 

 exopodite is divided by a transverse suture into two joints, but 

 the endopodite is unjointed. 



