PHYLUM ANNl'LATA 



457 



3. GENERAL ORGANISATION. 



The general form of the body in the 



Chsetopoda is cylindrical, but in many, e.g., 

 some members of the families Polt/noidce 

 (Fig. 371) and Amphinomidce, there is a very 

 considerable degree of dorso-ventral com- 

 pression. In most the body is very long in 

 comparison with its breadth ; but this is not 

 a universal rule, the length being in some 

 cases not more than five or six times the 

 breadth. The surface is marked out by a 

 number of more or less distinct annular 

 constrictions or impressed lines into a corre- 

 sponding series of segments or metameres, 

 which are usually very numerous, often some 

 hundreds in number, though in some cases 

 there are not more than from twenty to 

 These segments are usually very similar throughout the 

 of the body ; but in the Cryptocephala (Figs. 372, 373, 



FIG. 371. Polynbe seto- 

 sissima. Dorsal view 

 of entire animal, with 

 the pharynx protruded. 

 (After Quatrefages.) 



thirty, 

 length 

 379) there 

 may be two 

 or even more 

 regions dis- 

 tinguishable 

 from one 

 another by 

 the form of 

 the segments 

 and of their 

 appendages. 

 In the Oligo- 

 chaeta there is 

 a thickened 

 zone, the cli- 

 tellum, com- 

 prising some- 

 times only 

 one segment, 

 sometimes a 

 number. Each 

 segment, with 

 certain excep- 

 tions to be 

 noted p r e- 

 sently, bears 

 either a pair 



(I hit 



FIG. 372. A Serpulid (Oaleularia cti'S]titosa). Lateral view of animal 

 gg removed from its tube. <ilnl. abdomen ; br. branchkc ; <>!> oper- 

 cuhim ; th. thorax. 



