THE CRAYFISH 

 Phylum Arthropoda Class Crustacea 



I. BEHAVIOR AND EXTERNAL FEATURES 



Exercise 1. — General Activities. 



(a) These directions apply equally well to any of the common 

 species of crayfish of the Genus Cambarus and can be used with slight 

 modifications for the lobster, Homarus americanus. Watch crayfishes 

 in shallow pans of water, or in aquaria, and study their manner of 

 walking and swimming. Are they sensitive to touch? How acute is 

 their sense of sight? Do they respond to stimuli as well when re- 

 moved from the water? Are they able to right themselves when 

 placed dorsal surface down? 



(b) Place some powdered carmine in the water on each side of an 

 individual that is not moving about and determine the direction of any 

 currents. Watch carefully and try to ascertain what produces these 

 currents. Hold the crayfish out of water and note the bubbles pro- 

 duced at the anterior end of the animal. Selecting a small individual, 

 or one having a very clean shell, look on the outside of the body, just 

 above the base of the great claw and in line with the eye, and see if 

 you can detect a flickering motion beneath the semi-transparent shell. 

 You should be able to explain these observations when you have studied 

 the gill chambers described on pages 144-146. 



(c) Observe crayfishes in large aquaria containing stones and 

 other objects. Note their habits of concealment, modes of swimming, 

 reactions to other crayfishes and to small bits of meat dropped into 

 the water. Do they show a preference for particular places in the 

 aquarium? 



Exercise 2. — External Features. 



(d) The body of the crayfish is made up of segments, or somites, 

 each one of which bears a pair of jointed appendages. There are three 

 general regions of the body: the head, consisting of five somites; the 

 thorax, consisting of eight; and the abdomen, consisting of six. The 

 telson, or median part of the tail-fin, is not considered to be a somite. 

 In the abdomen the somites are distinct and movable ; in the remainder 



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