FOEM OF THE BODY OF ARTHROPOD A. 235 



as are the arguments against such a supposition, which are afforded 

 us by the differences presented by them in many points of their 

 organisation. At present therefore we can only with safety regard 

 the Crustacea as having a monophyletic origin ; this is indicated 

 by the common Nauplius stage. This stage, which is seen in all 

 the Entomostraca, obtains in some cases only among the Mala- 

 costraca, whence we may conclude that this latter division of the 

 Crustacea has a later point of origin than have the Entomostraca. 



The body of the Arthropoda, which is formed into metamores 

 either by gradual gemmation, or by the immediate differentiation 

 of the embryo, gradually loses, to a greater or less extent, the primi- 

 tive similarity of its segments. Great changes in external form are 

 brought about by the development of some, and the degeneration of 

 other, or, lastly, by the concrescence of a number of, metameres. 

 As a general rule the metameres are similar in the <^arly larval stages, 

 and so indicate relationship to forms in which tl^e metameres are 

 still indifferent. The more compact portions of the body, which are 

 formed by a fusion of the metameres, bear indications of their mode 

 of origin in the appendages which they carry. 



The most anterior metameres are those which most completely 

 undergo concrescence. In this way a portion which carries the 

 mouth and the higher sensory organs, and especially the eyes and 

 tentacles, is formed; this is the head. In the Myriapoda, many 

 Crustacea, and Insect-larvas, it is the only portion which is formed by 

 the concrescence of a number of metameres. Owing to this con- 

 crescence of the metameres the appendages are approximated to the 

 mouth, where they aid in taking up food, and are converted into 

 mouth-organs. Other differentiations play various parts in the 

 different divisions. In the Crustacea a number of metameres behind 

 the head unite with it to form a cephalothorax. The other meta- 

 meres are again often divided into two groups, inasmuch as those 

 behind the cephalothorax sometimes differ from the most posterior 

 ones. They thus form an abdomen and a post-abdomen. The 

 segments of the abdomen fuse in the Pcecilopoda, where the post- 

 abdomen is represented by the caudal spine. 



Special arrangements for the protection of the appendages 

 are formed by the folding and extension of the integument. Thus, 

 in the Decapoda, the dermal skeleton of the cephalothorax grows 

 out at the sides, covers the gills, and forms on either side a 

 special cavity — the branchial cavity — which communicates with the 

 surrounding medium. (Cf. § 187.) 



Such developments from the dermal skeleton, belonging to 

 several primitive body-segments, may extend over the other divisions 

 of the body, and form a iC shell " for them. The earliest form of 

 this is seen in the shield-like, enlarged, cephalothorax of the Phyllo- 

 poda (Apus) among the Branchiopoda. When the two halves of 

 this structure are further developed we get a bivalve shell (Fig. 

 124, cl) (Limnadia). In the Cladocera also a portion of the dorsal 

 integument is converted into a shell, which covers the whole of the 



