THE CRAWFISH. 119 



Preserve all these appendages in their proper order until 

 the dissection is finished. 



II. The Eye. Examine the eye. Pull it outward 

 with forceps, and note the length of its stalk. Its stalk 

 is muscular, and capable of turning the eye, when pro- 

 truded, to look in any direction, an admirable compen- 

 sation for the stiffness of the creature's neck. Examine 

 a section of its cornea with low power, to make out 

 whether it is a compound eye or an ocellus. 



III. Internal Organs. Now carefully dissect away 

 from the thorax the wall that is uppermost, and cut 

 away all of the carapace as far, at least, as the median 

 dorsal line, taking care not to injure any of the deli- 

 cate organs that lie beneath. Cut away, also, all the 

 part that is uppermost of the notum (tergum) of the 

 abdomen ; also the right pleurum. 



(1) Organs of Circulation. In the inner wall of the 

 branchial chamber, observe the vertical canals which 

 convey the blood from the gills toward the heart. These 

 canals may sometimes be seen through the wall before 

 dissection of this part. 



The heart lies in the top of the thorax, under the center 

 of the carapace, in a sort of membranous inclosure called 

 the pericardial sinus. AVhen the carapace is cut away, 

 beneath it should be found the thin, upper wall of this 

 cavity. Cut through this, and expose the heart, a thin 

 transparent, angular sac, suspended normally in the color- 

 less blood that fills the sinus. Find five small arteries 

 starting from its anterior end. Three of these, the central 

 ones, proceed directly forward toward the head ; and the 

 other two, the lateral pair, proceed downward and forward 

 toward the lobes of the liver, which they supply with blood. 

 Find a large artery starting from the posterior end of the 

 heart, and immediately dividing into two branches, one 

 of which continues backward along the upper side of 



