EXTERNAL CHARACTERS. 141 



pincers of the next two pairs of legs, and then cutting it up 

 into little bits with the jaws, which are placed at the sides 

 of the mouth. The real chewing is effected afterwards in 

 the stomach itself. 



The sexes are distinct. The male and female are very 

 similar, but the abdomen is broader in the female. 



The eggs are comparatively large, and are carried about by 

 the female, attached to the abdominal appendages, until the 

 young are hatched. The newly hatched crayfish differs con- 

 siderably in form from the adult, but has the full number of 

 appendages already present. It remains for a time attached 

 by its pincers to the appendages of the mother, but soon takes 

 to independent life. To allow for the growth of the animal, 

 the cuticle, which is incapable of interstitial growth, is cast 

 off periodically, and a new one secreted. This ecdysis, or 

 shedding of the cuticle, occurs three or more times during 

 the first year of the animal's life, and afterwards about once 

 a year, or less often. It involves not merely the external 

 investment of the body and limbs, but the covering of the 

 gills and sense organs, and to a certain extent the lining of 

 the alimentary canal. 



Crayfish may be killed almost instantaneously by drop- 

 ping them into boiling water. 



I. EXTERNAL CHARACTERS. 



The body of the crayfish is very obviously divisible into 

 an anterior un jointed portion, the cephalothorax, and a 

 jointed and flexible hinder portion, the abdomen. 



A. Examination of a Typical Segment of the Body. 



Examine in detail the third or fourth segment of the abdomen, 

 noticing the following 'points : 



1. The cuticular investment or exoskeleton of the segment 

 is hardened by calcification, except the portions at 

 the joints, which remain soft to allow of movement. 

 Each hardened portion is called a sclerite. 



M 



