i 4 o ELEMENTARY BIOLOGY. [CHAP. 



LABORATORY WORK. 



i. General external characters. 



The animal is covered by a dense exoskekton: in it 

 are readily recognised the following parts : 



a. The tody 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 proper and thorax: the anterior pro- 

 longation of the carapace to form the frontal 

 spine. 



P. The posterior segmented portion (abdomen): 

 its seven divisions ; the anterior six much like 

 one another; the most posterior (telson) 

 different from the rest. 



b. The great number of 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 by separating the append- 

 ages beneath the head. 



/?. The anus; a longitudinal slit beneath the 

 telson. 



y. The paired genital openings: in the male, on 

 the first joints of the last pair of appendages 

 of the thorax: in the female, on the first joints 

 of the last thoracic appendages but two. 



