i8 THE APODID/E part l 



that it grows directly out of the NaiipHus by the 

 gradual differentiation of new segments in front of 

 the anal segment, with no metamorphosis worth 

 mentioning. This fact has led to its being compared 

 with an overgrown Nauplius. According to our view, 

 indeed, the Nauplius is only the young Apus, or 

 Apus-stage in other Crustaceans. This steady develop- 

 ment of Apus from its larva, as an Annelid develops 

 from the Trochophora, falls in with our explanation 

 of its morphology. Apus, however, differs from its 

 Annelid ancestors in that it reaches its adult shape 

 before its inherited number of segments are fully 

 developed. This fixation of the hinder part of the 

 body at a larval stage can be easily accounted for by 

 the process of natural selection, compactness being a 

 decided advantage to an active free-swimming animal. 

 The great number of segments, developed and 

 rudimentary, in the Apodidae is a matter of 

 considerable importance in estimating their true 

 position. In all the other specialised Crustacean 

 groups the number of segments is constant, i.e. 

 constant for each group. In deriving such animals 

 with a small but constant number of segments from 

 Annelids with a large and varying number of seg- 

 ments, the ideal transition form would be an animal 

 with a medium number of segments, which is not 

 quite constant and is visibly diminishing. Both these 

 points are specially clear in the Apodid?e. We find 

 that all descriptions of Apus cancriformis give a 

 varying number of limbs, which can hardly be due 

 merely to the difficulty of counting them, but more 



