9 2 INVERTEBRATE ZOOLOGY. 



best studied in the following order. Remove them from one side only, leav- 

 ing those of the other in their natural order for comparison. 



1. A typical pleopod. Select this from the Bd-oth abdominal somite. It 

 consists of a basal piece, protopodite. double in this case : and of two 

 terminal branches, the oiiter or exopod.ite , and inner or endopodite. 

 This is considered a typical appendage, of which the others may be 

 considered modifications. 



3. The sixth pleopod. This is flattened to assist in the formation of the 

 tail. One of the terminal l)ranches is double. Which is it? 



3. The first and second pleopods. These are different in the two sexes. 



In the male they are large and peculiarly twisted, and capable of 

 approximation to form a copulatory tube for the conveyance of the 

 spermatozoa. In the female they are both normal in shape, but the 

 first are very small. 



4. ^4 walking leg ( periopod). Select the next to the last leg. Separate 



it from the body, taking care to remove it entire. It consists of 

 seven joints, to the first of which are attached a plumose gill and a 

 flat gill paddle, epijjodite or flahelhun. The joints are named in 

 order : (1) Coxopodite. (2) Basipodite. (3) Ischiopodite. (4) Mero- 

 podite. (5) Carpopodite. (6) Propodite. (7) Dactylopodite. Select 

 the first leg beyond the large claw. Notice that it ends in a claw 

 also i. e. is chelate {chela=c\aw). Does this result from the addition 

 of an eighth piece? Compare with the former one and explain. 

 Compare with the large claw. Is the difference in size the only one ? 

 cf. living animal for difference in use. Compare with the typical 

 form (a pleopod). The leg may be explained by the suppression 

 of one of the terminal branches and subdivision of the other into 

 joints. To find out which branch is suppressed, compare with the 

 next. 



5. The lliird maxilliped. This is the segment next anterior to the large 



chelae. (The segments will now be taken in order, going towards 

 the anterior end.) This is intermediate between a jaw and a foot, 

 as its name denotes. Has it an epipodite ? A gill V Do you recog- 

 nize the leg part ? Has it the same number of joints as the others ? 

 Do you find the branch which was not developed in the true legs ? 

 How woiild you then describe a periopod ? 



