54 GENERAL ZOOLOGY 



moulds and bacteria to gain an entrance. These are fought 

 off by little motile cells, the amoebocytes, until the wound 

 can heal. 



But a crayfish must struggle against other things besides 

 aggressive enemies and stealthy parasites. It. is obliged 

 to meet the changing conditions of environment so as not 

 to be destroyed by storms, or cold, or other unfavorable 

 variations. Crayfishes are resistant to low temperatures 

 and live a slow, yet active, life beneath the ice where there 

 is a winter season. They do not fare so well if the water 

 becomes too warm, and are therefore less abundant in the 

 tropics than elsewhere. They can live for a considerable 

 time out of water, particularly if they can rest in some 

 damp place, as among fallen leaves. 



Crayfishes quickly succumb to foul water and sometimes 

 migrate over the land to escape from a stagnant pool. If 

 the pond in which they have been living dries up, they 

 burrow into the soil and remain quiet until water is again 

 available. Some crayfishes have become expert diggers 

 and make burrows far from water; in fact, some enter the 

 water only during the breeding season. Such species often 

 do damage by making holes in fields or by perforating dykes. 

 They are easily killed by dropping unslaked lime down their 

 burrows and closing them tightly. 



Race Preservation. Crayfishes are of separate sexes. 

 Mature males are readily distinguished from females by 

 their larger chelipeds and narrower abdomens. The repro-*J 

 ductive gland, or testis, of the male lies in the posterior 

 portion of the thorax and is connected by two tortuous 

 ducts, the vasa deferentia, with small pores in the basal 

 joints of the last pair of walking legs. The first two pairs 

 of abdominal appendages in the male are peculiarly modi- 

 fied so that they may be used in transferring sperm to the 

 female. 



During the breeding season the males move about 

 actively in search of females. When one is encountered, 

 a little packet (the spermatophore) full of sperm cells is 



