APPENDAGES 



We may dismiss the uniramous type with a few words : it 

 is obviously secondarily derived from the biramous type ; this 

 can be proved in detail in nearly every case. Thus, the uniramous 

 second antennae of some adult forms are during the Nauplius 

 stage invariably biramous, a condition which is retained in the 

 adult Cladocera. Similarly the uniramous walking legs of many 

 Decapoda pass through a biramous stage during development, 

 the outer branches or exopodites of the limbs being suppressed 

 subsequently, while the primitively biramous condition of the 

 thoracic limbs is retained in the adults of the Schizopoda, which 

 doubtless own a common ancestry with the Decapoda. The only 

 Crustacean limb which appears to be constantly uniramous both 

 in larval and adult life is the first pair of antennae. 



We are reduced, therefore, to two types — the foliaceous and 

 biramous. Sir E. Eay Lankester,^ in one of his most incisive 

 morphological essays, has explained how these two types are 

 really fundamentally the same. He compares, for instance, the 

 foliaceous first maxillipede (Fig. 1, A), or the second maxilla 

 (Fig. 1, B) of a Decapod, e.g. Astacus, with the foliaceous thoracic 

 limb of BrancMpus (Fig. 1, D), and with the typically biramous 

 first maxillipede of a Schizopod (Fig. 1, F). 



In each case there is present, on the outer edge of the limb, 

 one or more projections or epipodites which are generally 

 specialised for respiratory purposes, and may carry the gills. 

 The 6 th and 5 th " endites " in the foliaceous limb (Fig. 1, D) 

 are compared with the exopodite and endopodite respectively 

 of the biramous limb, while the endites 4-1 of the foliaceous 

 limb are found in the basal joints of the biramous limb. 

 Lankester presumes that the biramous type of limb throughout 

 has been derived from the foliaceous type by the suppression 

 of the endites 1-4, as discrete rami, and the exaggerated 

 development of the endites 5 and 6, as above indicated. 



The essential fact that the two types of limb are built on the 

 same plan may be considered as established ; but it may be 

 urged that the biramous type represents this common plan more 

 nearly than the foliaceous. It is, at any rate, certain that in 

 the maxillipedes of the Decapoda we witness the conversion 

 of the biramous type into the foliaceous by the expansion of 

 the basal joints concomitantly with the assumption by the 



' Quart. J. Micr. Sci. xxi., 1881, p. 343. 



