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 Earthworms 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 surrounded 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 longitudinal 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, etc. 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 Acanthqodrilus (? 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 number of setae in each somite is also not constant. Among Earthworms Lum-bricus has a pair in each of the four longitudinal rows (i. e. eight) in each somite. In some species of Acanthodrilus, in Titanus, the eight setae are sometimes separated by equal intervals, and there are consequently eight rows, i. e. four on each side. The eight setae when thus separated may not remain in line from somite to somite, but alternate in position, as in the posterior somites of Urochaeta. Megascolex has very numerous setae, interrupted only in the median dorsal and ventral lines, whereas in Perichaeta and Perionyx single setae are implanted at intervals round the circumference of each somite. It is difficult to say whether the concentrated or scattered mode of distribution is the more primitive (see under nephridia, p. 205). The genus Acan-thodrilus affords an instance of variations in this particular in different species.
The clitellum of Earthworms appears to fulfil two functions. It is a copulatory organ, and in L. foetidus Perrier observed the formation of a resistent membrane from its surface during sexual congress. It also secretes the cocoon in which the ova are contained together with spermatozoa and albumen. The cocoon is stripped off forwards by contractions of the body: and is charged with its contents in transit (?). The two ends of the cocoon close of their own accord as soon as it is freed from the body. In some Earthworms the clitellum surrounds the body, and there is no ventral furrow, as occasionally happens in L. terrestris. Its position and extent are variable not only in different groups of terri-colous Oligochaeta, but within certain limits in the same genus and even the same species. In the aquatic Oligochaeta it includes only the somite in which the vas deferens opens, and it does not secrete during congress. It is apparently absent in Moniligaster among terrestrial Oligochaeta.
Phosphorescence has been noticed in various Lumbrici, and appears to be due to a fluid excreted from the hypodermis (see Vejdovsky, p. 67).
System und Morphologie der Oligochaeten, Vejdovsky, Prague, 1844. Id., Monographic der Enchytraeiden, Prague, 1879. Lombriciens Terrestres, Perrier, Nouvelles Archives du Museum d'Hist. Nat. de Paris, viii. 1872. Studies on Earthworms (first of a series), Benham, Q. J. M. xxvi. 1886.
Lumbricus terrestris. Claparede, Z. W. Z. xix. 1879; Horst, Tijdskr. Neder-land. Dierk. Vereen, iii. 1876; cf. A. N. 43, (ii), 1877, p. 481; Brooks, Handbook of Invertebrate Zoology, Boston, 1882, p. 140; Howes, Atlas of Practical Elementary Biology, London, 1885.
P/ute//us, Perrier, A. Z. Expt. ii. 1873. Urochaeta, Id. op. cit. iii. 1874. Pontodrilus, Id. op. cit. ix. 1881. Megascolex (= Pleurochaeta) Moseleyi, Bed-dard, Trans. Roy. Soc. Edinburgh, xxx. pt. 2. Megascolex, Perichaeta, Id. A.N.H. (5), xiii. 1884. Do. with Perionyx and Typhaeus, 'Earthworms from India,' Id. op. cit. xii. 1883. Perichaeta, Moniligaster, 'Earthworms from Ceylon,' Id. op. cit. xvii. 1886. Acanthodrilus capensis, Id. Proc. Roy. Soc. Edinburgh, 1885. Micro-chaeta Rappi, Id. Tr. Z. S. (to appear); and Benham, op. cit. supra.
Hypodermis. Von Mojsisovics, SB. Akad. Wien, lxxvi. Abth. i. 1877; Ray Lankester, Q. J. M. xx. 1880, p. 303. Body wall. Beddard, Proc. Roy. Soc. Edinburgh, 1884. Muscles. Rohde, Schneider's Zool. Beitrage, Breslau, i. 1885. Dorsal pores and musculature. Ude, Z. W. Z. xliii. pt. i. 1885. Peritoneal epithelium. D'Arcy Power, Q. J. M. xviii. 1878.
Cocoon and congress. Perrier, A. Z. Expt. iv. 1875, notes, p. xiii.; cf. Ratzel, and Warschawsky, Z W. Z. xviii. 1867-69, p. 547.
Phosphorescence in Earthworms. Vejdovsky, op. cit. supra, p. 67; Cohn Z. W. Z. xxiii. 1873. In Polychaeta. Panceri, Atti Acad. fis. mat. Naples, vii. 1878 (cf. Journal de Zool. v. 1876, p. 94); Robin, Bull. Soc. Philomath. (7), vii. In Polynoe. Jourdan, Z. A. viii. 1885. General account. Mc Intosh, Nature xxxii. 1885, p. 476; cf. Panceri, A. Sc. N. (5), xvi. 1872, and Secchi, on Spectrum, ibid. In decaying organisms. Pfluger, in his Archiv f. Physiol, xi. 1875.
Vegetable mould and Earthworms. Darwin, London, 1881; Hensen, Z. W. Z. xxviii. 1877. Turriform heaps. Trouessart, C. R. 95, 1882. Absence from N. W. Prairies of N. America. Nature, xxix. 1883-84. Habits. Ibid. xxx. 1884.