EXTERNAL ANATOMY OF THE CRAYFISH 1 (1) 



1. Examine a preserved Crayfish and note that the body, which is 

 entirely covered with a chitinous exoskeleton, consists of a rigid ante- 

 rior portion (cephalothorax) and a jointed, flexible, posterior portion 

 (abdomen). 



2. The portion of the exoskeleton covering the cephalothorax is known 

 as the carapace. The latter ends anteriorly in a dorsal projection 

 (rostrum). Examine the carapace and note the indentation (cervical 

 groove) in the exoskeleton which marks the division between the head 

 and thorax. Note : (a) the short antennules ; (6) the long antennae 

 with the opening of (c) a green gland, or kidney, on the basal joint of 

 each ; (d) the compound eyes ; and (e) the ventral mouth concealed by 

 appendages. 



3. The abdomen consists of six similar segments and, at the posterior 

 end, a median structure (telson) which is generally regarded as a seventh 

 abdominal segment. Note the anal opening on the ventral surface of 

 the telson. Examine the abdominal segments and note how they are 

 joined together so as to permit movement. A pair of biramous append- 

 ages is attached to each abdominal segment except the telson. In the 

 male, the first two pairs of abdominal appendages are modified and en- 

 larged for the transfer of sperm, while in the female the first pair is greatly 

 reduced. In the female, there is also a cavity (seminal receptacle) in 

 the mid-ventral line between the fourth and fifth pairs of walking legs. 

 Determine the sex of your specimen. 



4. Examine the exoskeleton of a detached abdominal segment and 

 note : (a) the dorsal, arched portion (tergum) ; (b) the ventral, calcified 

 bar (sternum) ; and (c) the projecting, lateral portions (pleura). Ex- 

 amine the attached biramous appendages and note : (tf) the basal portion 

 (protopodite) which is attached to the body and bears (e) two jointed 

 branches (exopodite and endopodite). 



5. Make (a) a drawing of the entire animal from the left side and (6) a 

 drawing of a detached abdominal segment from one end. 



6. For comparison, examine a number of other common species of 

 Crustacea, such, for example, as the tiny Water-flea (Daphnia) or the 

 one-eyed Cyclops, the sessile Barnacle {Balanus), the Crab (Callinedes 

 or Gelasimus), the Spider Crab (Libinia), and the terrestrial Pill-bug 



(Oniscus). (W. f. 51.) 



i B. pp. 99-100. 

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