PHYLUM ANNULATA 



429 



which characterise the genus and distinguish it from the many 

 other genera of the family Lumbricidcc : 



The prostomium is dovetailed completely into the peristomium. 

 The setaB are always in couples. The clitellum begins between 

 the twe.nty-sixth and the thirty-second segment, and extends over 

 six or seven segments. The male apertures are always on the 

 fifteenth segment. There are three pairs of vesicular seminales, 

 in the ninth, eleventh, and twelfth segments, connected across the 

 middle line in the tenth and eleventh by sacs enclosing the 

 ciliated funnels. 



The family Lumbricjdas is distinguished from the other families 

 of the sub-order Megadrili, which comprises all the Earthworms, 

 by the combination of the following features : 



The clitellum usually begins behind the twentieth segment and 

 occupies from six to nine segments; it is incomplete ventrally. 

 The male apertures are not situated further back than the fifteenth 

 segment. There are three or four pairs of vesiculae seminales, 

 in the ninth to the twelfth segments. The testes and ciliated 

 funnels are in the tenth and eleventh segments. The female 

 apertures are on the fourteenth. 



3. GENERAL ORGANISATION. 



The general form of the body in the Chastopoda is cylindrical, 

 but in many, e.g. some members of the 

 families Polynoidce (Fig. 336) and Amphi- 

 nomidce, there is a very considerable degree 

 of dorso-ventral compression. 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 corresponding 

 series of segments or metamercs, which are 

 usually very numerous, often some 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 Tubicolous Polychasta (Fig. 337) 

 there may be two or even more regions 

 distinguishable from one another by the 

 form of the segments and of their appen- 

 dages. In the Oligochaeta there is a thickened zone, the clitellum, 

 comprising sometimes only one segment, sometimes a number. 



Fio. 336. Polynde seto 



sissima. Dorsal view 

 of entire animal, with 

 the "pharynx protruded. 

 (After Quatrefages.) _, 



