COMPARISON WITH ANNELIDA. 315 



It is displayed in the metameric segmentation of the body, 

 the lateral extension of the segments into processes which sub- 

 serve locomotion, the presence of a ventral nerve cord surrounding 

 the oesophagus anteriorly and of a dorsal heart. We may 

 therefore begin our review of the characters of the Arthropoda 

 as a whole by drawing a comparison between them and the 

 Annelida. 



In the Annelida the body is covered by a soft and chitinous 

 covering. The mouth opens on the first segment of the body 

 and a prestomial lobe projects in front of it on the dorsal side. 

 Each segment contains, in the Polychaeta, a spacious coelomic 

 cavity, that of the first extending forwards into the prestomium.* 

 The supraoesophageal ganglion lies in the prestomial lobe, and 

 the first member of the ventral ganglionic chain behind the 

 mouth in the first segment (peristomial region). The pre- 

 stomium bears a pair of tentacular appendages (sometimes 

 with a median tentacle in addition), and the parapodia which 

 project at the sides of the body, although differentiated for 

 locomotory and tactile functions, are never jointed, and none 

 of them are modified as jaws to assist the introduction of 

 food into the mouth. The characteristic annelidan disposition 

 of the nephridia and gonads has been fully treated in an earlier 

 part of the present work and need not be here recapitulated. 



The cuticle in the Arthropoda is more rigid than that of the 

 Annelida and is generally hardened by a deposit of salts of 

 lime. It is secreted by a layer of cells, the epi- or hypo- 

 dermis, and from time to time during the period of growth 

 of the animal the hard outer layers of the cuticle are separated 

 from the inner layers, and ruptured, the animal emerging from 

 its cast " skin " (ecdysis). The soft inner layers then expand 

 to accommodate the growing body. 



Except in some cases, and usually in the posterior region, the 

 segments are produced laterally into limbs, and these, like the 

 body from which they spring, are divided into segments by 

 annular tracts of flexible cuticle intervening between the hard 

 and allowing movement between the successive segments and 

 of the whole limb upon the body. 



Movement is effected by a muscular system, which is contained 



* See, however, the remarks on this subject in the chapter on Annelida, 

 vol. I, p. 448. 



