COMMON EARTHWORM. 



199 



of the sacs in the ventral row. They appear to be special formations. Branched 

 pigment cells are found most plentifully in the connective tissue of the circular 

 layer of muscles. They are more numerous on the dorsal and lateral aspects of the 

 body than on the ventral ; more numerous in its anterior than in its posterior half. 

 The body-cavity is lined by a peritoneal epithelium which varies in character in 

 different regions and where it coats different organs. 



The septa dividing the body into compartments are composed of connective 

 tissue, of radial muscular fibres continuous with the fibres of the circular coat, and 

 of circular fibres surrounding the digestive tract, the supra-neural blood vessel and 

 nerve cord. Aeolosoma has only one septum separating the head from the rest of 

 the body. The most anterior septa in the common Earthworm are replaced by the 

 muscular bundles which pass to the pharynx from the body-wall. The perforations 

 in the septa, by which one division of the body-cavity communicates with another, 

 are in some instances near the body-wall. 



The divisions of the body-cavity communicate with the exterior in most Earth- 

 worms by dorsal pores, situated quite close to the anterior intersegmental furrow of 

 each somite. In the genus Lumbricus these pores commence in the sixth, seventh, 

 or eighth somite. They become occluded in the clitellum by the pressure of the 

 gland cells. They are simple perforations of the body-wall. Their aperture is sur- 

 rounded by a circular sphincter muscle, and is opened by an anterior and posterior 

 band of longitudinal fibres. The peritoneal cells at the margin covering the longi- 

 tudinal divaricators are massed in a small heap. The function of the pores appears 

 to be that of expelling coelomic fluid, or lymph. A cephalic pore is found in the 

 lower Oligochaeta except Aeolosoma, either on the ventral or dorsal aspects of the 

 prostomium, or else anteriorly and terminally. Dorsal pores are found in Enchytraeus, 

 Nais, &c. They are absent in Polychaeta. 



The setae are S-shaped, with the outer end more pointed than the inner. 

 They are implanted in sacs or trichophores, which are simple invaginations of the 

 hypodermis, composed of three (?) cells, one "basal and two lateral. New setae are 

 produced in the same sac as the old, and each seta is the product of a single cell. 

 The genital setae of the tenth to the fifteenth somites, of the twenty-sixth somite 

 and of the clitellum, are produced in the sacs of the ordinary setae present before 

 the worm becomes sexually mature. The sacs enlarge as soon as the ordinary setae 

 drop out, and from the enlarged cells are produced the long delicate genital setae. 

 The corresponding setae in the aquatic Oligochaeta are produced in new sacs, the 

 setae and sacs previously present undergoing atrophy (Vejdovsky). The setae of 

 Urochaeta are bifid at their apex : the genital setae of Acanthrodrilus (? all species) 

 and of Urochaeta, and the setae in general of Rhinodrilus, are variously ornamented. 

 Consequently the setae of terrestrial Oligochaeta are not invariably simple as Clapa- 

 rede supposed. The form of the setae in aquatic Oligochaeta is generally either simple 

 and hair-like, or else slightly curved with a bifid apex. In the genus Anachaeta the 

 trichophores are present but form no setae. The same thing occurs in Urochaeta 

 with certain setae. In Branchiobdella (if it is an Oligochaete) all traces of the sacs 

 are lost. Genital setae are always developed on the clitellum, and their form is 

 variable, but different as a rule to those of other parts of the body. 



The arrangement of the setae in two rows, an outer and inner or dorsal and 

 ventral, is the common one in Oligochaeta^ but it is sometimes departed from. The 



