SECT. Ill THE APPENDAGES 51 



organ attached to the hmb. We shall see further 

 reasons for this homology in the section on the 

 Nauplius. 



In later sections we shall see, further, that although 

 the ambulatory limb of the Decapoda has been 

 derived from the Phyllopodan limbs of their Lepto- 

 stracan ancestors, yet ambulatory limbs may develop 

 straight from Annelidan parapodia, as we assume to 

 have been the case in the Trilobites. The method of 

 differentiation is the same in both cases, but the fact 

 that the Decapodan limb first passed through a 

 Phyllopodan stage has made a slight difference in 

 the result (see section on the Trilobites). 



This brief discussion on the limbs of Apus and of 

 the Crustacea is by no means exhaustive. We have 

 purposely limited ourselves chiefly to our main point, 

 viz., how the limbs of Apus have been derived from 

 the parapodia of an Annelid. In so doing we have 

 naturally had our attention called to several homolo- 

 gies which may not at this stage appear altogether 

 satisfactory, but which will be found to hang together 

 with our whole argument. 



E 2 



