MOKPHOLOGY OF CYCLOPS. 



43 



the elongation of the parapodia for swimming mnst have demanded a firmer point d'appui, 

 and entailed the fusion of the anterior segments. That this has taken place may he 

 judged from the unsegmented Nauplius, which in the egg was markedly divided into three 

 segments, a point hitherto hut little noticed. The limhs would work to hetter effect 

 by being displaced ventrally, and the downward extension of the carapace into pleura, 

 very little marked indeed in some Copepods {Calamus), would probably aid the action of 

 the limbs by concentrating their action on a limited volume of water. But as in onto- 

 geny the segments develop from before backwards, and the limbs develop only slowly 

 and later than their proper segment, we should look for the disappearance of the abdo- 

 minal appendages by a mere retardation, not by progressive abortion — i. e.,pro tanto, a 

 retention of a young condition in that region, such as we find actually in the pelagic worm 

 Tomopteris ; and when once they disappeared in the adult, it would be vain to seek any 

 traces of them in the larva. That they should reappear does not imply a new creation 

 of parts, but just such an occurrence as the revival of suppressed parts, with which every 

 vegetable morphologist is familiar. In Nebalia the hindermost abdominal feet are united 

 by a median coupler (Claus) as in Copepoda. We are led, then, on all sides to the 

 conclusion that the Copepjoda represent a primitive type of Crustacea. 



How, then, would the Nauplius stand ? Purely as a necessary larval form, with its 

 anterior segments fused, and in no sense ancestral ; and this is the generally accepted 

 view. Beyond Copepoda we can imagine a series of forms with progressively disjointed 

 cephalic ends and rudimentary abdominal parapodia, and these would hardly be counted 

 as Crustacea at all. To essay a Phylogeny : — 



{Mesopida.) 

 Copepoda Natantia. 



C( pepoda Parasitica. 

 Eudiplopida. 



Eudiplopida stenopleura. 



(Braucliiura.) 



Eudiplopida bathypleura . 



Cirripedia. 



Rhizocephala. 



Ostracoda. 



Protophyllopoda . 



This table explains itself. By assuming that Cirripedia and Ostracoda come off from 

 the parent stock soon after the development of the compound eyes, we can understand 

 their inconstancy in the two groups— a recently acquired organ, like a new trick, being 

 first forgotten. 



One point more remains. If in any Crustacea we are to seek a common relation to the 

 Tracheata, especially to the Arachnida, it must be the Copepoda. While I do not think 



