166 HOMOLOGY 



attached to the protopodites which, in turn, are freely attached 

 to the body. On the fifth ambulatory protopodites and on the 

 internal surfaces, are the openings of the male organs of re- 

 production (Fig. 68,14 ). Here also the skin or membrane of the 

 protopodite is drawn out into a leaf-like organ termed the bract 

 or flabellum, a structure which reappears in each of the thoracic 

 appendages and serves as a partition wall between the gills in 

 the branchial chamber. All of the other ambulatory append- 

 ages are like the fifth in consisting of one shaft, the endopodite, 

 but on the protopodites of the first four in addition to the 

 bracts, there are outgrowths of membrane which form the gills 

 in the branchial chamber (Fig. 68, gill). The endopodites are 

 jointed, consisting of five parts or joints termed podomeres. 

 There is nothing in their structure to show that they are endo- 

 podites and not exopodites, this fact being established by em- 

 bryology, all of the thoracic limbs appearing first as biramous 

 appendages with both exopodites and endopodites (see Fig. 78). 

 The exopodites wither and disappear as growth progresses, leav- 

 ing only the endopodites. Similarly with the antennae, jaws 

 and antennules, the exopodites have disappeared or are so 

 highly modified as to be indistinguishable leaving only the inner 

 branches. The remaining appendages of head and thorax are 

 not so highly modified that homologous parts cannot be made 

 out although they must be studied part by part with the 

 principles of homology in mind. These parts are better seen 

 in the accompanying figures. 



All of these diverse appendages have been developed from the 

 primitive simple type of the biramous appendage of the abdomen 

 and well illustrate the principle of adaptation for particular 

 functions. The walking legs for example are adapted for this 

 means of locomotion and the anterior pair for offence and 

 defence; the jaws for crushing food; the maxillae and maxilli- 

 pedes for seizing, sifting and propelling food into the jaws. 

 It would seem as if unnecessary parts of the appendages had 

 disappeared leaving only those portions which are useful for 

 the purpose of the particular appendage. Many biologists 

 hold that such adaptations come through use or disuse of 



