70 LABORATORY WORK. 



are arranged.) Above the antennulse are the eyes. Are 

 they movable? Examine the black portion (cornea), and 

 see the small portions (facets) of which it is composed. 



Make a tabular arrangement of the appendages of the 

 body,* and ascertain by Savigny's law (p. 68) how many 

 somites there are in the body of the crayfish. Compare 

 the somites, and see how their diversity is brought about 

 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 man- 

 dibular somites, the line crossing its middle (the so-called 

 cervical suture) 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 specimen, 

 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 cervical suture, 

 is the oblong whitish heart. How many openings (ostia) 

 can you find through its walls? 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. 



* For reasons which cannot be discussed here, the eyes, although 

 jointed, are not regarded as appendages comparable to the others. 



