io8 LABORATORY MANUAL FOR ELEMENTARY ZOOLOGY 



series of segments, like the abdominal segments of the lobster, all bearing 

 biramous appendages. 



e) Comparative study of the appendages: The appendages will be studied by 

 comparing them with the typical biramous appendage described in the preceding 

 section, and determining what modifications have occurred. Consult Hegner, 

 Table VII (pp. 197-99). 



(1) The abdominal appendages: These are designated as swimmerets, since 

 they are employed in swimming. In the female they also serve as places of 

 attachment of the eggs (Hegner, p. 213). The swimmerets of the second to 

 fifth abdominal segments are very similar to the typical appendage already 

 described. The sixth pair of swimmerets, called the uropods, is greatly enlarged 

 and forms with the telson a powerful swimming organ, the tail fan. Determine 

 other differences between the uropods and the other swimmerets. The first 

 pair of swimmerets in the female lobster and crayfish is greatly reduced but 

 otherwise similar to the others. The first pair of swimmerets in the male lobster 

 and the first and second pairs in the male crayfish are modified for the transference 

 of the sperm to the female. They consist of protopod and endopod fused into 

 a hard, pointed, grooved structure. Exopod is lacking, except in the second 

 pair of the crayfish, where it appears as a soft, slender, lateral process. 



(2) The thoracic appendages: Remove the branchial region of the carapace 

 from the left side by lifting it up and cutting away the free portion. The branchial 

 chamber is thus exposed. Observe that all of the thoracic appendages have gill- 

 bearing processes extending up into the branchial chamber. The thoracic 

 appendages comprise five pairs of walking legs, called pereiopods, and three 

 pairs of smaller appendages, called maxillipeds, anterior to them. 



Remove completely the left third maxilliped, the appendage just in front of 

 the large pincers. Be sure to get the gill-bearing process with it. Cut through 

 the arthropodial membrane at the base and gently detach the appendage. The 

 basal joint, the coxopod, of the third maxilliped bears a delicate, hairy, leaflike 

 expansion, the epipod, to which a feathery gill is attached. Notice the great 

 freedom of movement of the coxopod and explain. The next joint distal to the 

 coxopod is the basipod. From it arise two branches, an inner endopod, consisting 

 of five joints, and an outer exopod, of many small joints. The maxilliped is 

 therefore a biramous appendage, similar to the swimmeret, but showing a process 

 of reduction of the exopod. The basal joint of the endopod has a row of hard 

 teeth, used in crushing food. 



Remove the left second pereiopod with all of its parts and compare with the 

 third maxilliped, placing both before you in the same position. The pereiopod 

 has a coxopod with an epipod and gill, a basipod, and an endopod of five joints. 

 The comparison shows, however, that the exopod is completely lacking, and the 

 pereiopod is therefore a uniramous appendage. Observe the pincer at the end 

 of the pereiopod and determine how it arose. 



