238 ANNULATA. 
called the Zeristomium 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 A/ollusca, but 
the body is enclosed in a thin cuticle or a thickened 
cuticular exoskeleton. 
The phylum is divided into two sub-phyla :—(1) ANNE- 
Lipa and (2) ARTHROPODA. 
Sus-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 (1) the 
metameric segmentation. The body has a great number of 
segments, usually preceded by a prostomium and a peri- 
stomium. The nervous, blood-vascular, ccelomic 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 (sete, suckers) and with the internal vas- 
cular coelomic fluids in the way described for Avenicola. (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.* 
Crass I.—ARCHIANNELIDA. From the type Poly- 
gordius it can be seen that this class contains the simplest 
* The Polycheta and Oligocheta are often placed together as Chetopoda, with 
the presence of setaze in comnion. 
