30 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAEUS 



chief locomotor organs. The chelae were held in readiness in front of the animal 

 and the second limbs (fig. 23) were bent inward under the body with the tip for- 

 ward so that in moving forward the body rode over these appendages as upon 

 levers with very little movement of the base of these appendages. The third 

 appendages always pointing forward swung from an angle of about 45 with 

 the side of the body to one of about 70. The fourth limbs had a very long 

 swing from a forward position about 45 with the side of the body back past 

 90 with the side of the body. The fifth limbs had a very short, hobbled, 

 movement like the third but always directed backward, from about 100 to 

 120 with the side of the body. 



In taking up the appendages of the second larva, in sequence (figs. 25 to 

 39) it will be noticed that the relative nakedness of the first larva has given 

 place to a hirsute condition, indicated in figures 23 and 24 ; setae occur upon the 

 antennae, chela?, legs, pleopods as well as the sides of the abdomen and, as the 

 separate sketches show, upon all the other appendages. The active second 

 larva has thus come into possession of sensory and locomotor setae lacking in 

 the imperfect first stage, and similar to those of the adult stages. 



The first antenna (fig. 25) still has the same general form as in the 

 first stage (fig. 6), it has five segments in exopodite and in endopodite but it is 

 noticeably more finished in being well armed with setae. The narrow part of 

 the proximal segment bears a sharp spine upon its inner side. The auditory 

 pit on the basal segment is now well guarded by a row of plumose setae pass- 

 ing from the outer edge inward and spread across the orifice of the pit. There 

 is also a row of sparsely branched plumes along this upper face of this segment 

 and parallel to its inner edge and in addition there are a few other setae ar- 

 ranged as in figure 25. The second and the third segments bear long plumose 

 setae on their inner sides and long, stiff spine-like setae on the external sides of 

 their distal ends. The endopodite a-nd the exopodite each bear a few long stiff 

 spine-like setae at the distal ends of their five segments and in addition the char- 

 acteristic blunt sense clubs of this appendage are now evident. These organs 

 are placed in groups upon the inner and lower faces of the third, fourth, and 

 fifth segments of the exopodite. The third segment has a cluster of three upon 

 its distal edge, the fourth has a group of two at its distal edge while the fifth 

 has a group of three at its middle part, where its diameter suddenly diminishes. 

 As all these sensory clubs face downward they are foreshortened in the above 

 figure and in reality are much longer than shown. 



The second antenna (fig. 26) has increased greatly in length over its former 

 state (fig. 7). Its basal parts are more angular and the excretory cone on the 

 basal segment is relatively very much smaller while the spines of the second 

 segment are borne upon a large, scale-like protuberance. The exopodite scale 

 bears a row of long, plumose setae all along its outer edge while in the first 



