24 THK YOUNG OF THE CRAYFISHES ASTACUS AND CAMBARUS 



but five segments in its exopodite and in its endopodite, is but embryonic com- 

 pared with the adult that has at least thirty-five in its exopodite and almost as 

 many in its endopodite, a difference which is of great moment when we con- 

 sider the repetition of special sense organs that are found on many successive 

 segments of the exopodite of the adult. The auditory organ also is apparently 

 of no functional value in the early larva. The number of segments in the fila- 

 ment of the second antenna must also increase greatly to form the one hun- 

 dred and twenty-five of the adult; an increase apparently brought about by in- 

 terpolating new segments at various places by division of the old ones into 

 two. 



While all the adult appendages of the head-thorax are represented in the 

 larva at hatching, this is not the case for the abdomen, for the sixth pair of ab- 

 dominal appendages are not externally present and the first pair which in the 

 adult male are so essential are absent in the first larva as they also are in the 

 adult female. The other abdominal appendages are four pairs of simple ple- 

 opods which hang down beyond the pleural plates of the abdomen so that they 

 are seen from a side view (fig. 3). Each pleopod (fig. 39) is as in the adult 

 composed of a short and a long segment that make up the protopodite and of 

 two simple terminal plates, the endopodite and the exopodite. These are 

 slightly curved and armed at the ends and to some extent on the edges with 

 small weak spines and they entirely lack the plumose setae that makes them use- 

 ful in the adult for fanning the water. The illustration is of the anterior face 

 of the left pleopod of the second abdominal segment and shows that the ex- 

 opodite is longer and wider than the endopodite, while in the adult the exopodite 

 is much the shorter and smaller in the female pleopods that bear the eggs and 

 in the male pleopods that transfer sperm. The appendages of the sixth somite 

 though not externally free are yet present and of large size though imperfect 

 in development and lie within the s-ubstance of the telson, as can be seen in 

 transparent living larvae. It is their presence which swells out this region 

 ventrally and gives rise to the protuberance seen from the side view (fig. 3). 

 Looking at the telson from above (fig. 20), the very imperfect future sixth ple- 

 opods are seen as two somewhat less translucent areas, right and left in the 

 anterior part of the telson and each having an outline suggesting that of a mit- 

 ten. Only later will these concealed buds of the sixth appendages burst out 

 after a moulting and expand as the very large lateral parts of the caudal fan, 

 so essential in quick locomotion. This retention of these appendages within 

 the telson in Astacus was known to Huxley ('80), and figured by Eeichenbach 

 ('86). 



The telson requires special consideration in connection with the "telson- 

 thread," as mentioned above. The telson itself is a very large plate with 

 nearly circular outline and is thin posterior to the above region occupied by 



