160 ELEMENTS OF COMPARATIVE ZOOLOGY. 



by under- development (atrophy) of one part and over- 

 development (hypertrophy) of another. (The carapax is 

 really but the dorsal portions of the antennal and mandib- 

 ular somites, the line crossing its middle being the line of 

 union of these two.) 



Make a side view of the crayfish, twice the natural size, 

 naming the parts. 



INTERNAL STRUCTURE. 



The dissection should be made under water, the speci- 

 cimen, back upwards, being held in position by being 

 pinned to the wax bottom of the dissecting-pan, the pins 

 passing through the telson and large claws. Open the 

 crayfish along the back by cutting away the carapax with 

 the scissors, taking care not to injure the underlying parts. 

 Continue the cuts backward, removing the upper surface of 

 the abdomen. 



Just beneath the carapax, behind the impressed line 

 crossing it, is the oblong whitish heart. How many open- 

 ings through its walls can you find ? How many tubes 

 (arteries) leading from it ? With the forceps gently tip 

 the heart to the side. Can you find more openings or 

 more arteries ? Is there a chamber (pericardium) around 

 the heart ? Trace the arteries as far as you can without 

 injuring other parts. 



Beneath the heart, and projecting from beneath it, are 

 the paired reproductive organs. Do those of the two sides 

 connect ? Can you find the ducts leading down from 

 them ? Where do they end ? Still farther in front is the 

 large thin-walled stomach, and on either side of this, and 

 reaching back to the heart, is the liver, reddish in the 

 crayfish, green in the lobster. Tip the stomach back- 



