I9O ELEMENTARY BIOLOGY. [CHAP. 



LABORATORY WORK. 



Crayfish are best killed under chloroform, proceeding as 

 with the Frog. (p. 35). They may be best kept alive in a 

 moist atmosphere near running water, fed upon sopped 

 bread. A sink, covered with a sheet of glass, meets all 

 requirements. 



A. General external characters. 



The animal is covered by a continuous exoskeleton which 

 is for the most part calcified, the following parts are 

 readily recognised: 



a. The body proper : 



a. Its anterior unsegmented portion (cephalotho- 

 rax): the great shield-like plate (carapace) 

 covering the back and sides of the cephalotho- 

 rax; the groove across the carapace (cervical 

 suture) marking out the line of junction of head 

 and thorax: the anterior prolongation of the 

 carapace to form the rostrum or frontal spine. 



ft. Its posterior segmented portion (abdomen): its 

 seven divisions; the anterior six much like one 

 another; the most posterior (telson) different 

 from the rest (Cf. pp. 170 and 174). 



b. The jointed limbs (appendages) attached to the 

 ventral aspect of the body : their varying characters 

 in different regions. 



c. The external apertures of the body. 



a. The mouth; seen on turning aside the append- 

 ages beneath the head. 



