238 ANNUL AT A. 



called the peristomimn which differs in many respects from 

 the segments behind it. In the great majority each segment 

 carries a pair of appendages which may be parapodia, 

 legs, jaws, and so on. 



The nervous system consists of a dorsal brain in the 

 prostomium, a circumoral ring round the front end of the 

 gut and a double ventral nerve-chain with or without 

 ganglia. 



The heart, when present, is dorsal to the alimentary canal 

 and may show traces of segmentation. 



There are never true " shells," as in the Mollusca^ but 

 the body is enclosed in a thin cuticle or a thickened 

 cuticular exoskeleton. 



The phylum is divided into two sub-phyla: — (i) Anne- 

 lida and {2) Arthropoda. 



Sub-Phylum I. — Annelida. 



The Annelida is the sub-phylum of segmented worms, 

 and in anatomical characters it is sufficiently definite. The 

 most diagnostic characters of the sub-phylum are (i) the 

 metameric segmentation. The body has a great number of 

 segments, usually preceded by a prostomium and a peri- 

 stomium. The nervous, blood-vascular, coelomic and 

 excretory systems are mostly repeated in the segments. 

 (2) The nervous system always consists of a dorsal brain 

 in or near the prostomium, a nerve-ring in the peri- 

 stomium, and a long ventral chain, usually more or less 

 segmented and showing a double origin. (3) The muscular 

 system and chief method of locomotion are quite character- 

 istic. The circular and longitudinal muscles, contained 

 in a tough, flexible body-wall, work in conjunction with 

 external organs (setae, suckers) and with the internal vas- 

 cular coelomic fluids in the way described for Arenicola. (4) 

 Highly developed nephridia are not confined to the sub- 

 phylum, but are very characteristic of it. 



The four classes are intimately connected by inter- 

 mediate types but can hardly be further approximated.'^ 



Class I. — Archiannelida. From the type Poly- 

 gordius it can be seen that this class contains the simplest 



* l:\i& PolychcBta 2a\A. Oligochceta are often placed together as Chcstopoda, with 

 the presence of setaae in common. 



