170 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 repro- 

 duction (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 endopodites and 

 not exopodites, this fact being established by embryology, 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, leaving only the en- 

 dopodites. 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 re- 

 maining appendages of head and thorax are not so highly modi- 

 fied 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 well shown in the accompanying 

 figures. 



All of these diverse appendages have been developed from the 

 primitive simple type of the biramous appendage of the abdo- 

 men, 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 de- 

 fence ; the jaws for crushing food ; the maxillae and maxillipedes 

 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 parts, the useless por- 



