PHYLUM ANNULATA 



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 Polynoulce 

 (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- 

 FIQ. 37i.-poiynbe seto- spending series of segments or metameres, 

 sissima. Dorsal view which are usually very numerous, often some 



of entire animal, with , . r J/L i_ 



the pharynx protruded, hundreds in number, though in some cases 

 there are not more than from twenty to 



thirty. These segments are usually very similar throughout the 

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

 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 OHgo- 

 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 pre- 

 sently, bears FIG. 372. A SerpuUd (Galeolaria coespitosa). Lateral view of animal 

 removed from its tube. abd. abdomen ; br. branchiae ; op. oper- 



eitner a pair culum ; t h. thorax, 



abet 



