PHYLUM AJSTNULATA 



467 



region of the enteric canal is eversible as a proboscis, and there are 

 usually horny jaws. 



There are several species of the genus Lumbricus, differing from 

 one another in the general form of the body, the number of the 

 segments, the shape of the prostomial lobe, and other minor 

 points. All of them agree in the presence of the following features, 

 which characterise the genus and distinguish it from the many 

 other genera of the family Lumhricidce : — 



The prostomium is dovetailed completely into the peristomium. 

 The setfe are always in couples. There are longer and straighter 

 setse on the clitellum. The male apertures are always on the 

 fifteenth segment. There are three jjairs of vesiculte seminales, 

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

 middle line in the tenth and eleventh by sacs enclosing the 

 ciliated funnels. There are two pairs of receptacula seminis al- 

 ways situated in the ninth and tenth segments. 



The family Lumbricidce 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. 

 Dorsal pores are present. The setse on the clitellum differ from 

 the others. The male apertures are not situated further back than 

 the fifteenth segment. There are three or four pairs of vesiculse 

 seminales, in the ninth to the twelfth seg- 

 ments. The testes and ciliated funnels are ^"-^'^a 

 usually in the tenth and eleventh segments : 

 the female apertures on the fourteenth. 



3. General Organisation. 



The general form of the body in the 



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

 some members of the families Polynoidm 

 (Fig. 365) and Amphinomidw, 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 

 metameres, 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 



H H 2 



Fifi. 365.— Polynbe seto- 

 sissima. Dorsal view 

 of entire animal, "with 

 the pharynx protruded. 

 (After Quatrefagos.) 



