NO. 545] THE BIOLOGY OF THE CRAYFISH 



2S7 



scuttle under her abdomen. It is probable that here the 

 visual sensitivity to moving objects is more highly 

 developed than in the adult in comparison with other 

 senses. 



The young crayfish moults very frequently during the 

 first year. 



I found that two or three days before moulting the adult 

 crayfish come up into the shallows exposing their cara- 

 paces and drying them out thoroughly. The first time that 

 I saw this prolonged drying-out process I did not think it 

 significant, for I have seen crayfish in ponds where the 

 water was pure and fresh, elevating their carapaces for a 

 few minutes at a time. It is a habit which is not neces- 

 sarily caused by impure water, for the same thing was 

 noted by me in the laboratory with animals in the run- 

 ning-water tank. 



When I noted by the number (in oil paint) on its back 

 that the same individual was continually remaining only 

 partly submerged, I made a note and watched develop- 

 ments. Later, in three other crayfish I noted this pre- 

 liminary drying out, and predicted the approximate time 

 of the moults. This was convenient knowledge, for a 

 crayfish in difficulty with his half removed old coat falls 

 easy prey to his brethren. 



It is possible that the aeration of the attached young by 

 the mother is for the purpose of enabling the young cray- 

 fish to moult more readily. Observations like these have 

 not been reported for other crayfish or for marine crusta- 

 ceans, but it seems possible that in the crayfish we have 

 such a drying out of the old exo-skeleton as we find taking 

 place in insects, like the dragon fly, which live for a time 

 in the water. 



Andrews made a thorough study of the young of both 

 Astacus and Cambarus and found that in Cambarus the 

 young four months old averaged about 41 mm. in length. 

 During the winter of the first year there is no increase in 

 size, but the second summer of life marks an increase of 

 thirty per cent, in length (12). 



