THE CRAYFISH 241 



iostegite and part of the side of the body underneath it are 

 thin. On the ventral side of the cephalothorax the limbs 

 of each pair are close together, but small sterna lie between 

 them. The head is, of course, the region which contains 

 the mouth and the principal sense organs. The mouth is 

 placed on the ventral surface at some distance from the 

 front end, and in front of it the sternal surface slopes up- 

 wards to the rostrum. At the sides of the latter, upon a 

 pair of short, movable stalks, are placed the eyes, and below 

 these stand two pairs of feelers or antennae. 



The limbs or appendages number nineteen pairs, without 

 counting the eyes, which are by some authorities 

 reckoned as limbs. We shall not take this 

 view, but as there is evidence in the development of the 

 crayfish and of related animals that the foremost region of 

 the head corresponds to a segment, we shall regard the 

 body as containing twenty segments, of which the foremost 

 bears no limbs. The telson is not a segment. Of the 

 twenty segments, the first six form the head, the next 

 eight the thorax, and the last six, with the telson, the 

 abdomen. The parts of which a complete limb consists 

 are best seen in the limbs known as the third pair of 

 maxillipeds (Fig. 156, A), which lie immediately in front 

 of the great pincers. In each of these we may distinguish 

 a two-jointed basal region or protopodite, the joint nearest 

 the body being known as the coxopodite, the next as 

 the basipodite. On its outer side the coxopodite bears 

 a large, flat, thin-walled structure known as the epipodite, 

 covered with small finger-processes, which constitute a 

 gill. At the base of the epipodite is a knob bearing 

 a tuft of threads known as the coxopoditic seta or 

 setobranch. The basipodite bears two structures. From 

 its outer side arises a slender, jointed appendage known' as 

 the exopodite. At its end, continuing the axis of the limb, 

 is the stout, five-jointed endopodiie, whose joints, starting 

 from the basipodite, are known as the ischiopodite, meropo-' 

 dite, carpopodite, propodite, and dactylopodite. The other 

 limbs are built upon the same general plan as the third 

 maxilliped, but in detail the structure of every one of them is 

 adapted to the particular work it has to perform, each part 

 being of a different shape in each pair of limbs, or sometimes 



