x PHYLUM ANNULATA 417 



segment or chamber of the coelome. As the two mesoderm 

 segments become closely applied to one another and unite around 

 the alimentary canal, their two cavities also come into close 

 relation, and eventually are separated from one another only by 

 thin vertical septa, which afterwards form the dorsal and ventral 

 mesenteries. Successive mesoderm segments also come into close 

 relationship with one another, their cavities eventually only 

 remaining separated by thin transverse partitions, which form the 

 intersegmental septa or mesenteries. 



The region in front of the prototroch becomes modified to form 

 the prostomium of the adult. The body increases in length, and 

 additional segments with their setigerous sacs become distinguish- 

 able (E) until, on the development of the tentacles, the outgrowth 

 of the parapodia (para.} with their cirri and the permanent seta3 

 (which replace those first formed), the formation of the full number 

 of segments, and the completion of the internal organs, the adult 

 condition of the worm is attained. 



b. THE EARTHWORM (Lumbricus). 



General External Features. The Earthworm (Fig. 328) 

 has a long narrow body, which may be described as approximately 

 cylindrical, but slightly depressed towards one end, the posterior. 

 Dorsal and ventral surfaces are readily recognisable, the latter 

 being much paler in colour than the former, and exhibiting a 

 slight flattening ; an 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 segmtents, 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 which is the opening of the mouth. Next to 

 the prostomium is the most anterior segment, the pyp/istomium, 

 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 seg- 

 ments from the thirty-second to the thirty-seventh, has a swollen 

 appearance ; this is termed the clitdlum. There are no parapodia 

 like those of Nereis, but running along the lower surface of the 

 worm are to be recognised with the aid of a lens four double rows 

 of short bristles or seta3 (Fig. 329), a pair of each row occurring 

 in each segment, which thus possesses eight altogether. The 

 extremities of all these setae are directed backwards, and they act 

 as fulcra for the forward movements of the worm on the surface 



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