144 THE CRAYFISH 



of the body they are covered dorsally and united by the carapace into 

 a rigid portion, the cephalothorax. Note the pointed rostrum between 

 the stalked eyes. Compare anterior and posterior ends, dorsal and 

 ventral surfaces, and right and left sides. Is there any departure 

 from strict bilateral symmetry? Examine the paired appendages from 

 anterior to posterior end. Is the entire animal covered by the exoskel- 

 eton? AVhat is the nature of the skeleton at the joints of the body 

 and of the appendages? After looking carefully at the proportions of 

 the abdomen, draw (XI) an outline of an ideal cross section through 

 this region to show the shape of the dorsal and ventral parts of the 

 shell and the shape and attachment of the appendages ; do not actually 

 cut across the specimen. 



(e) Find the mouth bounded by lateral toothlike jaws and the 

 anus on the telson. Note the excretory pores on the bases of the 

 longer feelers, or antennae. Find on the dorsal surfaces of the smaller 

 feelers, or antennules, the clear flat areas which mark the position, of 

 the statocysts, or organs of equilibrium. Examine the bases of the 

 walking legs for the genital pores, or openings of the reproductive 

 ducts; they are found on the last pair in the male, and on the second 

 from the last pair in the female. In the male of Cambarus the two 

 first pairs of abdominal appendages are modified to form, when pressed 

 together, a copulatory organ along which the spermatozoa pass after 

 leaving the genital pores. What is the structure of the corresponding 

 appendages of the female? In the female there is a slitlike seminal 

 receptacle on the ventral surface between the bases of the last pair 

 of walking legs. Place a male and a female side by side and note 

 the difference when they are viewed dorsally. 



II. RESPIRATORY ORGANS 



Exercise 3.— The Gills. 



(a) Note the extension of the shell from the back down over the 

 bases of the walking legs to form the carapace. Lift up the free ven- 

 tral edge of the carapace and see the spongy mass of the gills, or 

 branchiae. Taking care not to injure the gills and using your scissors, 

 remove the carapace from the left side, thus exposing the full ex- 

 tent of the gill cavity. Do not cut too far dorsally and injure other 

 organs. Cut off the four walking legs, or pereiopods, and the large 

 claw, or cheliped, of this side a short distance from their bases. Sub- 

 merge the specimen in water in a dissecting pan and, by floating up 

 and carefully parting the mass of the gills, determine what a single 

 gill is like and where it is attached to the body. Move the stumps of 



