HOMARUS 169 



mology with true appendages is doubtful, there are nineteen 

 pairs. These are, counting from before backward: anten- 

 nules (or first antennae), antennae, six pairs of mouth appen- 

 dages, five pairs of walking legs (pereiopods) , of which the 

 first are the claws or chelae, and six pairs of swimmerets 

 (pleopods) . In the male, the first two pairs of pleopods are 

 modified to form copulatory organs. The first pair is greatly 

 modified and the second pair bears a special portion. 1 



(a) Turn one of the fifth pair of pleopods forward and 

 examine its posterior aspect. It consists of a basal piece, 

 the protopod; a lateral branch, the exopod; and a median 

 branch, the endopod. This branched type of appendage is 

 designated as biramous. What is its use? Compare with 

 this the modified sixth pair of pleopods, called the uropods. 



Make a drawing of one of the fifth pleopods. 



(b) In front of the chelae will be seen the sixth pair of 

 mouth appendages, the third maxillipeds. Remove that of 

 the right side and compare it with the fifth pleopod. In ad- 

 dition to the protopod, exopod, and endopod, it bears a long 

 blade, the epipod, which extends into the gill chamber. The 

 protopod is composed of two segments, coxopod and basipod; 

 the endopod of five segments, ischipod, meropod, carpopod, 

 propod, and dactylopod. The exopod is composed of one 

 long and many short segments. How is the appendage 

 modified to serve in feeding? 



Make a drawing of the third maxilliped. 



(c) Remove the remaining five mouth appendages and 

 compare each with the third maxilliped. These are, begin- 

 ning posteriorly, the second maxilliped, first maxilliped, sec- 

 ond maxilla (with a broad paddle, the scaphognathite, the 

 use of which should be understood), first maxilla, and the 

 mandible. Just back of the mandibles are two small flaps, 

 the paragnatha, which are not true appendages. Do you 

 understand the use of each of these appendages? Most of 

 the appendages have parts that may be compared with the 



1 The crayfish has the first two pairs, both greatly modified. 



