SECT, in THK APPENDAGES 29 



and swimmerets of the same animals, the ventral 

 parapodia on these limbs disappearing entirely. 



We shall further find that this division of labour 

 in the modern Crustacea was not discovered by 

 Nature all at once. Many different combinations of 

 the ventral parapodia as jaws, with dorsal parapodia 

 as auxiliary appendages, held their own for long 

 periods. In the long run, however, the typical 

 Crustacean formula for mandibles and maxilla: has, 

 except in Limulus and in the Ostracods, which have 

 different masticatory formulae, alone survived. These 

 efforts of Nature to select the best arrangement of 

 ventral and dorsal parapodia for the transformation 

 of a carnivorous Annelid into an armoured Crustacean 

 will be found tabulated in Part II. p. 250. 



Before describing the limbs of Apus in detail, 

 we must call attention to several important points, 

 which tend to support their claim to have originated 

 from Annclidan parapodia in the way described. 



(i) The limbs of Apus are little more than highly 

 developed integumental folds with only rudiments 

 of articulations, either between the different joints 

 of the shaft and its appendages, or between the 

 shaft and the body (Figs. 4 and 5). This absence 

 of developed articulations has already been pointed 

 out by Lankester and others, but its true signifi- 

 cance does not seem to have been noticed. It 

 is true that in many small thin-skinned typical 

 Crustaceans the articulations seem to be slightly 

 developed, but in these the whole exoskcleton has 

 been reduced ; this does not affect the significance 



