20 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAKUS 



basal and next segment of the protopodite there are a few blunt spines as in- 

 dicated in figure 6. This also shows the auditory organ as an open pit on the 

 upper surface of the basal segment, in the part that is swollen out laterally. 



The second antenna (fig. 7), though it has a long filament, is still embryonic 

 in proportions. The base consists of two short segments, the first of which 

 bears the very large prominence that faces inward and has the opening of 

 the green gland, or kidney, within its depressed top. The second segment 

 bears a few blunt distal spines of large size. The exopodite is a very large 

 flat scale ending in a blunt point and bearing some sixteen blunt spines along 

 its serrated inner and anterior edges. The endopodite consists of three large 

 basal segments and of a long round filament of many segments, forty-five to 

 fifty, the first of which is long and slender, while the following ones are each 

 about one-third as long as the first. The terminal segments are again more long 

 and slender. As indicated in figure 7, there are a few spines at the distal edges 

 of some of the terminal segments and of some of the others near the tip. In 

 the adult there may be 125 segments. 



The mandibles (fig. 8) have a well developed cutting edge which, however, is 

 smooth and not toothed as it is in all later, functional, stages and there was 

 no evidence found that these organs were used. The palpus has three segments 

 and is smooth except for the distal face of the third segment which is sparsely 

 set with rather acute spines, many of which are shown in figure 8, and more 

 of which are present upon the inner aspect and not seen from this point of 

 view. The palp is thus a blunt club with terminal spines. 



The first maxilla (fig. 9) is very small and made of two flat plates and a 

 somewhat rounded and blunt endopodite of curved finger-shape. Here for the 

 first time we meet with a few small, plumose seta; along the outer edge of the 

 distal segment. The two flat plates that represent the protopodite are spinulous 

 at the ends, much as in the palp of the mandible. The distal piece is also armed 

 with a row of a few spines along its proximal edge as seen in figure 9. The 

 ends of both plates are set with spines on the face toward the mouth; the 

 proximal plate is rounded, the distal one truncated to form a jaw-like organ. 



The second maxilla (fig. 10) bears the long scaphognathite which has a 

 dense row of plumose setae all along its extensive free edges. There are also 

 a few plumose setae at the base of the endopodite, as in the first maxilla (fig. 

 9). As above noted one of the first muscular activities acquired after hatching 

 is the slowly developed rhythmic beat of the scaphognathite, and with this use 

 of this appendage there is present an armament of plumose setae lacking else- 

 where in the locomotor organs of the larva. While these plumose setae are not 

 used in locomotion their function as flexible areas of resistance to the water 

 which the scaphognathite bales out of the branchial chamber is akin to that of 

 locomotor setae. The rest of the second maxilla is nearly bare of setae, but there 



