34 THE YOUNG OF THE CRAYFISHES ASTACUS AND CAMBAKTJS 



shorter of the two flat lobes that bear setae so that the adult relationship in 

 size of these parts was not yet arrived at. 



While the telson of the first larva was nearly circular, in the second it was 

 swollen laterally (fig. 23), and became thus transversely elongated since the 

 sixth pleopods had now grown within it as very large lateral masses. The 

 posterior edge of the telson was somewhat incised on the median plane and 

 thus recalled the early bilobed condition found in the embryo some days be- 

 fore hatching. 



The long plumose setae of the telson (fig. 23) that aid the larva in swim- 

 ming are the expanded products of the radiating glands seen within the telson 

 of the first stage (fig. 20). 



Thus provided with effective swimming setae and more numerous sensory 

 setae the second stage larva gradually depends less and less iipon its mother 

 and finally leaves her altogether. After some eight to ten days these active 

 larvae cast off their shells and passed into a third stage. 



The third larva was in the main very like the second but it had advanced 

 a very important step in freeing its sixth abdominal appendages which hence- 

 forth are not inside the telson, but lying by the side of it to make the effective 

 tail-fan that is used in rapid locomotion. Some hours before shedding the sec- 

 ond larva plainly showed the sixth pleopods as dark red, partly opaque masses 

 within the base of the telson and after shedding these appendages were ex- 

 panded as is shown in figures 40 and 42. As both the end of the telson and the 

 edges of these great flat sixth pleopods are set with plumose setae the combined 

 fringe of setas augments the surface used by the larva in escaping backward 

 by vigorous blows of the telson and sixth pleopods against the water. 



The details of this effective and very large sixth pleopod which has been 

 forming slowly on each side within the base of the telson ever since the larva 

 came out of the egg, that is from some two to three weeks, are shown in figure 

 42, which shows the dorsal face of the left appendage of. the sixth abdominal 

 somite. This appendage joins onto the sixth somite and lies by the side of the 

 telson as indicated in figure 40. The protopodite bears a prominent spine over 

 the base of the endopodite ; the endopodite is armed with two spines near its 

 edge and the exopodite with five spines, along the edge of the two segments into 

 which it is divided, as in the adult, by a movable hinge. Scattered over the sur- 

 face are a few relatively short acicular setae. It will be noted that the plumes 

 along the edges of both endopodite and exopodite are arranged to make a most 

 effective fan since those of the endopodite overlap some of those of the exopodite, 

 when, as in figure- 42, the exopodite is not extended as far as possible away 

 from the median plane. At times the exopodite may be shut in under the endop- 

 odite like a part of a fan. 



The real length of the terminal plumes is somewhat greater than shown in 



