INTERNAL ANATOMY OF THE CRAYFISH 1 



1. Secure a good-sized preserved Crayfish and place in dissecting pan. 

 Hold the animal firmly with one hand, dorsal surface up, and insert the 

 points of the scissors under the center of the posterior margin of carapace. 

 Beginning at this point, cut through the carapace in the mid-dorsal line 

 and continue the cut to the base of the rostrum, stopping just back of the 

 eyes. From this point make transverse cuts through the carapace to the 

 right and to the left. Each cut should extend ventrally from the mid- 

 dorsal line to the ventral edge of the carapace, just posterior to the eyes 

 and at the base of the antenna. The right and left halves of the cara- 

 pace, which have now been completely severed, may be carefully detached 

 from the underlying tissues and discarded, thus exposing the internal 

 organs of the thorax and head regions, including the lateral gill chambers. 



2. Note the delicate pigmented membrane which, if unbroken, covers 

 the internal organs. Remove the membrane and note : (a) the thin- 

 walled stomach in the midline near the anterior end of the body ; (b) the 

 muscular heart lying in the pericardial cavity, the walls of which 

 consist of (c) a delicate pericardium. Find : (d) the large anterior 

 median artery which extends forward from the anterior margin of the 

 heart ; and (e) the dorsal ardominal artery which extends posteriorly 

 through the abdomen. On either side and below the heart note : (/) a 

 digestive gland ; and (g) a gonad just anterior. Carefully remove the 

 heart, taking care not to disturb the other organs. Examine it under 

 water and find (h) three pairs of openings (ostia) through which the blood 

 enters the heart from the surrounding pericardial cavity. 



3. Examine the stomach and locate : (a) the short esophagus, run- 

 ning dorsally from the mouth and opening into the ventral surface of the 

 stomach at the anterior, or cardiac portion. The digestive glands open 

 into the posterior, or pyloric portion of the stomach. Trace (b) the 

 intestine posteriorly from the stomach to the abdomen. With the scis- 

 sors cut through the entire length of the abdominal wall in a mid-dorsal 

 line. Separate the cut edges and find (c) the intestine lying between 

 the abdominal muscles : trace it to the external opening on the ventral 

 surface of the telson. Make a drawing of the alimentary canal to show 



iB. pp. 104-112. 

 291 



