EXTERNAL ANATOMY OF THE CRAYFISH 1 (2) 



1. Carefully cut off a portion of the carapace from the left side of your 

 specimen, thus exposing the left gill chamber containing the gills. 

 Then lay the animal on its right side under water. Note the feathery 

 character of the gills and their attachment either to the appendages or to 

 membranes present at the base of the appendages. Find the modified, 

 paddle-shaped portion (scaphognathite) of the second maxilla, which 

 lies in the anterior end of the gill chamber and by its movements keeps 

 a current of water bathing the gills. 



2. Examine and identify the 19 pairs of appendages present on the 

 Crayfish, using the figures and description in your textbooks. In doing 

 this it will be well to begin at the posterior end of the animal and work 

 forward. The appendages may be summarized as follows : 



(a) Abdominal Appendages, six pairs, namely, the uropod and five 

 pairs of swimmerets. The first two pairs of swimmerets are not typical 

 and are different in the two sexes. 



(6) Thoracic Appendages, eight pairs, namely, five pairs of walking 

 legs and three pairs of maxillipeds. 



(c) Head Appendages, five pairs, namely, two pairs of maxillae, one 

 pair of mandibles, one pair of antennae, and one pair of antennules. 

 (W. fs. 64, 65.) 



3. Beginning again at the posterior end of the abdomen, carefully 

 remove all the appendages one by one from the left side of the animal and 

 pin each one in a dissecting pan in its proper order and position. In 

 removing them, be sure to get the whole of each appendage, including the 

 gills when present. The small appendages around the mouth must be 

 handled very carefully with fine forceps. 



4. Study and make a drawing of the following appendages : (a) an- 

 tenna ; (6) SECOND MAXILLA ; (c) THIRD MAXILLIPED ; (d) FIRST and 

 SECOND WALKING LEGS, and (e) THIRD and SIXTH ABDOMINAL APPENDAGES. 



Identify and label the homologous parts in each. 



1 B. pp. 100-104. 



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