PHYLUM ANNULATA 



455 



slight flattening ; the anterior end is distinguishable in the living 

 animal as that which is directed forwards in the ordinary creeping 

 movements of the worm. The surface, as in the case of Nereis, 

 is very distinctly marked out into segments or metameres by a 

 series of ring-like constrictions ; the segments, which are very 

 numerous amounting to about 150, are somewhat longer towards 

 the anterior end than they are further back. 



At the extreme anterior end is a rounded lobe, the prostomium, 

 immediately behind and below which is the opening of the mouth. 

 Next to the prostomium is the most anterior segment, the peri- 

 stomium, which bounds the mouth behind. The eyes and tentacles 

 present in Nereis are not represented. On the most posterior 

 segment, the anal segment, is a small median opening, the anal 

 aperture. A limited region of the body in front of the middle, 

 comprising segments from the thirty-second to the thirty-seventh, 

 has a swollen appearance ; this is termed the 

 clitellum. There are no parapodia like those 

 of Nereis, but running along the lower sur- 

 face of the worm are to be recognised with the 

 aid of a lens four double rows of short bristles 

 or setae (Fig. 358), a pair of each row occur- 

 ring in each segment, which thus possesses 

 eight altogether. The extremities of all these 

 setce are directed backwards, and they act as 

 fulcra for the forward movements of the worm 

 on the surface of the ground or in the interior 

 of its burrow. The setae in the clitellum, and 

 those in the neighbourhood of the genital 

 apertures, are much slenderer than the rest. 



,, J i 11 T c ,1 i i f 



Along the middle line ot the dorsal surface, 

 from about the eleventh segment backwards, 

 is a row of small aperture's, one at the line of division between each 

 contiguous pair of segments : these, which are termed the dorsal 

 pores, perforate the body- wall and open internally into the coelome. 

 Through these ccelomic fluid is capable of being discharged, 

 covering the surface with a thin layer which may protect the 

 worm from desiccation or from contact with irritating sub- 

 stances. On the ventral surface are two rows of minute 

 apertures a pair on each segment the excretory apertures or 

 nephridiopores. On the ventral surface of the fifteenth segment 

 (Fig. 357, 15), is a pair of slit-like apertures with somewhat 

 tumid lips, the male reproductive apertures; and on the segment 

 immediately in front the fourteenth, are two smaller rounded 

 apertures, the female reproductive apertures. In the intervals 

 between the ninth and tenth, and tenth and eleventh segments 

 are two pairs of small pores, the openings of the receptacula 

 seminis. 



G. 358. Lumbricus. 

 Seta*, highly magnified. 



