160 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



For this influence, which is shown by such phenomena as the 

 inheritance by the child of polydactylous feet from the polydactylous 

 hands of the parent, or the development of wing-feathers on the toes of 

 fantail pigeons; the influence which has carried the feet of the horse 

 family along the same line of evolution with the hands, I have, in another 

 place, proposed the term ontogenetic inheritance. Among the arthropods, 

 examples of this sort of modern metamerization are very common, both 

 as normal features of their structure, in the movable body-rings of the 

 ocular and antennary somites of stomatopods, for example, and as 

 monstrosities, as in the twelve-legged coleoptera. 



I believe that a thorough study of this most interesting and instruc- 

 tive class of facts will convince any one that there is no philosophical 

 necessity for assuming that the primitive crustacean had a highly meta- 

 merized body like that'of a phyllopod, and that all the common features 

 in the structure of arthropods may have been derived from a common 

 ancestor as simple as a nauplius. 



The analogy between the parapodia of annelids and the limbs of 

 Crustacea has been held to prove that the primitive crustacean limb was 

 not a rowing organ fitted for a pelagic life, like the limbs of the nauplius 

 and the copepod, but flat and leaf -like and adapted for movement over 

 the bottom. 



It is hardly possible, however, to believe that the arthropods have 

 been derived from the higher polychaetous annelids, and as the simpler 

 and more primitive annelids have no parapodia, the resemblance, which 

 is not actually very noteworthy, can be nothing more than an analogy. 



There are plenty of degenerated copepods, and we have in their 

 structure abundant proof of the degeneracy, and an adequate explanation 

 of it in their parasitic habits, but they are degenerated descendants of 

 ordinary swimming copepods, and not of phyllopods, and there is no 

 reason for holding that the copepodan type itself is degenerate, except 

 the supposed exigencies of morphological philosophy. 



The active locomotor habits of the eucopepods of the open ocean 

 would seem to be conducive to advancement rather than to degenera- 

 tion, and the occurrence of phyllopods in the lower Cambrian is, of 

 course, no more evidence that they are primitive Crustacea than the 

 occurrence of pteropods and gasteropods is that they are primitive 

 molluscs. 



I am unable to see any valid objection to the view that the copepods 

 are primitively pelagic ; that they have been evolved at the surface of 



