234 



COMPARATIVE ANATOMY. 



Fig. 117. Nauplius of a 



Copepod (Cyclops), a h c 



Appendages. 



Form of the Body. 



§ 183. 



Among the Crustacea tlie simplest stage of tlie Arthropod body- 

 is seen in the Nauplius-form (Fig. 117). 

 The unsegmented body carries several pairs 

 of appendages. The body only becomes 

 segmented by a gradual process of gem- 

 matiouj which has many points of resem- 

 blance to the process which brings about 

 metamerism in most of the Annulata. The 

 most anterior portion of the body of the 

 Nauplius which carries the first appendages 

 forms the cephalic segment; the posterior 

 part becomes the last metamere ; new meta- 

 meres are formed between these two, on 

 which appendages similarly sprout. In this 

 way an organism composed of a larger 

 number of metameres is gradually formed (Fig. 118); the com- 

 plication of which, so far as it is due to metamerism, is the product 



of a gradual process. 

 This development of the 

 form of the body pre- 

 dominates in the Ento- 

 mostraca; it corresponds, 

 probably enough, to the 

 phylogeny of these Crus- 

 tacea, which therefore 

 may be referred back to 

 an unsegmented condi- 

 tion. IntheMalacostraca 

 this process of the forma- 

 tion of new metameres 

 can be made out in a few 

 cases only, and the rudi- 

 mentary body ordinarily 

 consists at the very first 

 of a larger number of 

 metameres. The con- 

 secutive formation of 

 metameres is here com- 

 pressed ; and the same 

 hajapens in the Poecilo- 

 poda and in most Tra- 

 cheata. Although this might seem to remove any doubts as to 

 the common origin of the Arthropoda, it is not of so much weight 



rig. 118. Larva of Br an chip us stagnais. 

 (second stage), ale Appendages, mx Rudiments 

 of the maxillae. J" Caudal fork, o Eye (after C. Clans). 



