PHYLUM ARTHROPODA 



109 



Examine the other pereiopods, comparing them with the second one and 

 noting differences. Do all have pincers, epipods, gills? By moving the legs, 

 note how the gills are moved. In what directions can the legs be moved upon 

 the body? In what directions can the joints of the leg be moved upon 

 each other? 



The first pair of pereiopods is greatly enlarged, with a powerful pincer, or 

 chela. Note that the two chelae of the lobster are not alike, but one is massive 

 with broad crushing surfaces, called the cracker claw, and the other sharper and 

 more slender, with little teeth, known as the toothed claw. 



On the inner side of the coxopod of the third pereiopods of the female find 

 the female genital openings. The male genital openings are in the same place 

 on the fifth pereiopods. 



Draw the third maxilliped and the second pereiopod in the same positions. 



Next remove the left second maxilliped complete. Compare with the third 

 maxilliped. Does it have the same parts? Notice reduction of the epipod and 

 gill and the presence of only four joints in the endopod. 



Remove the first maxilliped, and compare with the other two. Does it have 

 an epipod? gill? Observe particularly that the appendage has become more 

 broadened and leaflike. This is due to the moving of the coxopod and basipod 

 from their original basal position to form the medial side of the appendage. The 

 endopod is thus shoved laterally. Endopod and exopod of the first maxilliped 

 are slender processes of about equal size, the exopod resting in a groove of the 

 endopod. Endopod is thus gradually being reduced, consisting now of but two 

 segments, and the two segments of the protopod are being gradually broadened 

 and shifted to the inside. There is thus produced the foliaceous type of 

 appendage. 



The thorax therefore has eight pairs of appendages, the three pairs of maxil- 

 lipeds and the five pairs of pereiopods, and consists of eight segments. 



(3) The head appendages: The head has five pairs of appendages, omitting 

 the eyes. These are, beginning with the most posterior, two pairs of maxillae, 

 a pair of mandibles, the antennae, and the antennules. 



Examine the second maxilla in place. Does it have an epipod? gill? Observe 

 that the epipod is continuous with an anterior process, which is the exopod, the 

 whole forming an elongated blade, pointed at both ends, which fits into the 

 anterior end of the branchial chamber. This blade, the bailer or scaphognathite, 

 moves back and forth, drawing a current of water through the branchial chamber 

 over the gills from the posterior end of the chamber forward. Within the exopod 

 is a slender endopod, still further reduced and consisting of but one joint, and 

 within this the expanded protopod, with four processes. The second maxilla is 

 thus decidedly foliaceous. Remove it and draw. 



The first maxilla is a reduced foliaceous appendage. Its two inner thin plates 

 are the protopod ; the outer slender process is the endopod, exopod being absent. 



