THE YOUNG OP THE CRAYFISHES ASTACUS AND CAMBABUS 33 



for the first time provided with the phenomenally long hair-like coxopoditic 

 setae found in the adult and in all but the first larval stages on this and all 

 the following thoracic somites. As seen in figure 33, these setae arise external 

 to the exopodite and add a conspicuous element to the appendages as they are 

 longer than the entire exopodite and coil about like stiff wires. The endopodite 

 bristles with exceedingly long, sharp needle-like setae and has no real plumes 

 whatever, though with 2 D" it was evident that some of the longest needles on 

 the terminal segment were very finely barbed. On the anterior, and inner, 

 face of that segment there were also numerous stout setae with flat saw-teeth 

 like those above described, on the corner of the penultimate segment of the 

 endopodite of the second maxilliped. 



The chela (fig. 34) has greatly increased in size, as shown in comparing 

 with fig. 14, and now has simple acicular setae scattered over it but no plumose 

 sete. The terminal claw is no longer used as a holdfast and has no longer 

 recurved tips. Henceforth of use as a cutting shears it now bears a large tooth 

 on each blade as well as the rows of spines formerly present. At the base of 

 the appendage the gills have increased in size and there is a long tuft of coxo- 

 podite setae, which, however, are short in proportion to the enormous endopo- 

 dite. 



The four walking legs (figs. 35, 36, 37, 38) though much larger than in the 

 first stage (figs. 15, 16, 17, 18) have not increased as much as have the chelae 

 and they retain their relative proportions and sizes. Like the chelae they now 

 bristle with acicular setae and show no plumose setae. However, at the distal 

 edge of the penultimate segment of the fourth and of the fifth legs (figs. 37, 38) 

 there are a few of the saw-like "cleansing setae" previously referred to as oc- 

 curring in the adult. The gills have changed only in size; the coxopodite set;e 

 are very long threads, but only few in number and on the last leg reduced to 

 one. 



The branchial formula in the second stage was thus just like that above 

 given for the first stage. 



On the abdomen of the second stage there were still but four pairs of ple- 

 opods since the first were not yet formed and the sixth still remained inside of 

 the telson. But each pleopod was now so well provided with plumose setae 

 that the appendage simulated a locomotor organ. The larvae also now had the 

 adult habit of swinging the pleopods back and forth and so producing currents 

 in the water which may well be of aid in respiration as they would change the 

 water supplied to the inhalent openings of the gill chambers. Each pleopod 

 (fig. 39) had grown greatly in length as compared with its first appearance 

 (fig. 19) ; the long plumose setae arose from the distal parts of the exopodite 

 and endopodite and resembled the plumes upon the exopodites of the maxillipeds 

 (figs. 31, 32, 33). The exopodite was still the longer and the endopodite the 



