374 



INVERTEBRATE MORPHOLOGY. 



jointed axis ending in a rounded lobe and bearing upon its 

 inner edge six lobes (1-6), some of which are united to the 

 axis by a joint. On the outer side are two large lobes, the 

 distal one being termed the nabellum (fi\ while the proximal 

 one is the bract (br) and serves respiratory purposes. The 

 entire appendage has thus a leaflike form. In one of the 

 more anterior appendages, however, an interesting modifica- 

 tion of this will be found. Thus in the second thoracic ap- 

 pendage (Fig. 166, B) the axis 

 will be found to be more dis- 

 tinctly divided into two joints, 

 each bearing two of the internal 

 lobes somewhat reduced in 

 size, while the terminal one in 

 addition carries two other lobes, 

 the fifth and sixth, which have 

 become somewhat elongated. 

 The flabellum and bract remain 

 nearly the same as in the 

 first post - genital appendage. 

 If now such an appendage be 

 compared with the second tho- 

 racic appendage of the Shrimp 

 Palcemonetes (Fig. 167), a direct 

 homology of the parts may be 

 discovered. The axis of the 

 Phyllopod limb is represented 

 by the protopodite while the 

 exopodite (ex) and endopodite 

 (en) represent the two terminal 

 inner lobes, the others having disappeared; the flabel- 

 luni is represented by the epipodite (ep) and the bract per- 

 haps by the branchia (br), attached to the epipodite in this par- 

 ticular limb, but free on the more posterior ones. 



It would appear probable from these facts that the bira- 

 mous limb is really a derivative from the more complicated 

 foliate appendage possessed by the Phyllopods ; the foliate 

 condition, however, has given place to such a great extent to 



FIG. 167. THE SECOND 

 FED OF Palcemonetes. 

 br braucbia. 

 en = endopodite. 

 ep = epipodite. 

 ex exopodite. 



