It projects into the latter (Bergh). The female apparatus is completed by two pairs of vesicular sperma-thecae which open in the intersegmental furrows between somites nine and ten, and ten and eleven, and are derived as in aquatic Oligochaeta from invaginations of the hypodermis (Bergh). They receive the spermatozoa in congress and secrete a clear fluid, which, according to Vejdovsky (op. cit. pp. 154-5), forms the spermato-phores. The clitellar glands are usually supposed to form these structures. The spermatophores are 1 1/2-2 mm. long and 0.5-0-7 mm. wide, somewhat spirally coiled, and with an open hollow at one end in which the spermatozoa lie parallel to one another. In some worms they are closed. They are usually attached on or about the clitellum.
The testes of Microchaeta are four in number as in Lumbricus, and are contained in the vesiculae. The organs in Earthworms usually described as testes appear to be the vesiculae. Of these there are generally two pairs; in certain forms, however, only a single pair, which extends through three somites in Urochaeta and Typhaeus, through twelve to fifteen in Titanus, and thirty or more in Urobenus (Benham). There is always a single pair of ovaries, which in Plutellus are situated in front of the testes. The ovaries may be lobed, e. g. in some species of Peri-chaeta. There are four separate vasa deferentia in Moniligaster and Acanthodrilus, each with its own aperture: and they each appear to possess, as does the vas deferens of the aquatic Oligochaeta, a terminal vesicle homologous with a nephridial vesicle. The ciliated funnels are in front of the vesiculae in Pontodrilus Marionis, while Anteus is stated to possess no sperm-ducts at all. The genital apertures are sometimes approximated ventrally and the oviducal aperture is single, and median in Perichaeta, Perionyx, and sometimes in Megascolex. The position of the male apertures with reference to the clitellum varies, and has been used by Perrier as a means of classifying Earthworms. The apertures in question are situated (1) in front of the clitellum, - Ante-clitelliani, e.g. Lumbricus: (2) within it, - Lntra-clitelliani, e. g.
Anteus, Titanus, Urochaeta: (3) behind it, - Post-clitelliani, e. g. Perichaeta, Megascolex, Plutellus, Pontodrilus, Acanthodrilus, Peri-onyx: (4) or there is no clitellum even in sexually mature specimens, - Aclitelliani, e. g. Moniligaster. Megascolex has been termed by Beddard Infra-clitellian, for though the vasa deferentia open within the clitellum, yet they occupy a non-glandular ventral area. As to the other accessory organs of generation, the setae of the male somites are sometimes much enlarged, especially in the genera Acanthodrilus and Eudrilus; copulatory papillae are present in some species of Perichaeta; and prostatic glands opening into the sperm-ducts are found in the worms just named. The spermathecae are always in front of the vesiculae except in Microchaeta and Eudrilus. They vary in number and size even in the same genus, and are sometimes complicated as in Perichaeta Houlleti by the addition of glands. There may be more than two spermathecae in each somite: in this case they are arranged in transverse rows.
See Beddard, A. N. H. (5), xvii. 1886, pp. 91-92 and 93-94: Benham, Q. J. M. xxvi. pp. 263, 280.
In the aquatic Oligochaeta the nephridia atrophy away in the somites in which the sexual glands and ducts are undergoing evolution. In Lumbricus and many other Earthworms the two sets of structures co-exist. Vejdovsky is inclined to suppose that the larger size of the coelome is the cause of this persistence. The evolution of the genital ducts in the Earthworms remains to be worked out 1 In many aquatic Oligochaeta it is accurately known. The vas deferens has in them a funnel, and a ciliated tube derived from peritoneal epithelium, and an atrium which is invaginated from the hypodermis like the nephridial vesicle, but is not represented in most Earthworms. The spermathecae are invaginations of the same kind. The oviducts are either simple slits (Aeolosoma, Chaetogastridae, etc.) or a pair of funnels (Lumbriculidae), and are obviously degenerate. Vejdovsky is inclined to conclude (op. cit. p. 161) that the sexual ducts either correspond to all the parts of an ordinary nephridium (vasa deferentia of aquatic Oligochaeta, of Acanthodrilus and Moniligaster) or to some of them (spermatheca to the nephridial vesicle: the oviduct of all, and vasa deferentia of Lumbricus, etc. to the nephridial tube): and that they are excretory organs developed only at the time of sexual maturity.
In the lower Oligochaeta the sexual apparatus atrophies after the sexual products are discharged, but the worms appear to live on and may propagate themselves asexually. It is probable that the development of the sexual apparatus of the Earthworms will be found to be similar to that of the aquatic Oligochaeta. The only thing that must be regarded as certain at present is the fact that the sexual ducts of Oligochaeta are not, as in many Polychaeta, ordinary nephridia, which take on a sexual function at the reproductive season. It is possible that the resemblances traceable between them and the nephridia are merely homoplastic1 and not homogeneous: i. e. the structures may not be identical. And the fact that the vasa deferentia of the Earthworms extend through several somites before they open externally may be held to support this view. It is true that the nephridia of the aquatic Phreatothrix extend through several somites. They form, however, U-shaped tubes and their external and internal apertures are always in two contiguous somites as in other Oligochaeta (Vejdovsky, op. cit. p. 124).
1 Bergh states that he intends to publish on the subject.
The terrestrial Oligochaeta do not multiply by fission and gemmation as do many of the aquatic forms. They appear, however, to have considerable powers of regeneration: and can form new anterior and posterior somites, supra- and sub-oesophageal ganglia, and pharynx in the place of parts removed by excision. They have been occasionally found with a bifid posterior extremity.
Digestive tract. Claparede, op. cit. ante, p. 600; in general, Vejdovsky, op. cit. pp. 100-112. Chloragogen cells, Vejdovsky, op. cit. pp. 110-112. Secretion of calcigerous glands, Robinet, C. R. 97, 1883. Digestive ferments. Krukenberg, Unter-such. Physiol. Inst., Heidelberg, ii. 1882, p. 37; Fredericque, Bull. Acad. Belg., (2) 47, p. 217. Colouring matter from intestine. MacMunn, Proc. Philos. Soc, Birmingham, iii. 1881-3, p. 389.
Vascular system. Jaquet, Mitth. Zool. Stat. Naples, vi. 1885; Vejdovsky, op. cit. pp. 112-120. Double dorsal vessel. Beddard, Proc. Roy. Phys. Soc, Edinburgh, 1885. Blood and blood corpuscles. Vejdovsky, op. cit. p. 118; Ray Lankester, Q. J. M. xviii. 1878, p. 72; Blomfield and Bourne, Q. J. M. xxi. 1881. Endothelium of vessels. D'Arcy Power, Q. J. M. xviii. 1878. Coelomic, i. e. amoeboid cells. Kukenthal, J. Z. xviii. 1885.
Nephridia. Claparede, op. cit. ante, p. 615; Gegenbaur, Z. W. Z. iv. 1853. In general and development. Vejdovsky, op. cit. pp. 120-129. Of Acanthodrilus multiporus. Beddard, A. Sc. N. (6) xix. 1885. Of a N. Z. Acanthodrilus. Id. Z. A. viii. 1885.
Sexual organs. Hering, Z. W. Z. viii. 1856-57; Bergh, Z. A. ix. 1886. In general and development. Vejdovsky, op. cit. pp. 125-151. Development of spermatozoa, vesiculae seminales. Blomfield, Q. J. M. xx. 1880; cf. Jensen, Archives de Biol. iv. 1883. Spermatophores. Fraisse, Arb. Zool. Zoot. Inst., Wurzburg, v. 1882; Vejdovsky, op. cit. p. 154. Accessory organs. Vejdovsky, op. cit. p. 135. Homologies. Id., op. cit. pp. 15 7-161.
Regeneration of excised parts. Miss Fielde, Proc. Acad. Nat. Sc. Philadelphia, pt. 1, 1885; cf. on lumbriculus, Bulow, Z. W. Z. xxxix. 1883. Bifid Earthworms;
1 For 'homoplasy,' see Ray Lankester, 'Use of the Term Homology,' etc, A. N. H. (4) vi. 1870.
Bell, A. N. H. (5) xvi. 1885. Regeneration in marine Chaetopoda. De Quatrefages, A. Sc. N. (3) ii. 1844, p. 100; Claparede, Annelides du Golfe de Naples, 1868, p. 30. For lit. of subject in general, see Milne Edwards, Lecons sur la Physiologie et l'Anatomie compare'e, Paris, viii. 1863, p. 301 et seqq.; or Fraisse, Die Regeneration von Geweben und Organen bei der Wirbelthieren, Cassel und Berlin, 1885; cf. Horst, Z. A. ix. 1886.