Suspended by the anterior extremity to show its external characters.
The anterior region of the body tapers to a conical point: the posterior is flattened dorso-ventrally and tapers abruptly. The first somite socalled or prostomium (infra) lies entirely in front of the mouth and corresponds to the praeoral somite in other Chaetopoda. The mouth is ventrally placed in the second segment and is therefore subterminal. The anus on the other hand is posterior and terminal. The whole body is distinctly divided into a series of somites or segments, separated by well-marked intersegmental furrows. Vertical transverse septa or dissepiments which divide the body into compartments, correspond internally to these furrows with the exception of the first five or six. The compartments communicate with one another round the supra-nervian blood-vessel. The chitinoid cuticle has been loosened by maceration in the spirit and may be seen lying in folds in the posterior region of the body. In life it is iridescent owing to the presence of two sets of fine parallel superficial lines crossing each other at angles of 750 to 8o°. On the ventral surface of the fifteenth somite counting the praeoral as the first, are two white tumid elevations. These correspond to the apertures of the vasa deferentia.
If the worm be held so that the light falls upon it obliquely, two somewhat raised lines may be seen running down the body on each side parallel to one another. The more ventral of these lines corresponds very nearly in position with the apertures of the vasa deferentia. Both lines indicate the position of the setae which constitute the locomotor apparatus of the Chaetopoda. There are in each somite of the common Earthworm two setae in each line, implanted a small distance apart, but they are often lost by accidents. The outer row is wanting in some instances as far back as the clitellum (infra). The inner row usually commences on the fourth or fifth somite. Both rows may be absent in the posterior somites. The spot corresponding to the inner or ventral row on the twenty-sixth somite of this specimen is somewhat swollen. The setae are here, as in the region of the clitellum, peculiarly long and delicate in a sexually mature worm, and are generally retracted. They are supposed to act as accessory copulatory organs.
The dorsal and lateral parts of somites thirty-one to thirty-eight, and especially of somites thirty-three to thirty-six, are white and swollen, and the swollen region is bordered by a prominent well-marked edge on either side of the median ventral line. These swollen somites constitute the clitellum, an organ especially characteristic by its great development of the terrestrial Oligochaeta. It is glandular and secretes a plentiful mucus from which the cocoon is formed. The prominent ventral edge acts as a copulatory organ. The development of the clitellum depends on the age and the state of sexual activity of the individual.
Vejdovsky believes, from his observations on the growth of the prostomium or praeoral somite in the individual produced by fission of Aeolosoma tenebrarum as well as in embryoes of Rynchelmis, that it is really an outgrowth of the buccal somite and not a separate somite. In Aeolosoma the buccal somite developes the supra-oesophageal ganglion, the two cephalic provisional nephridia, and the pharynx, the prostomium growing forwards by degrees. It is not separated by a furrow from the buccal somite as it is in all other Chaetopoda except Chaetogastridae (see Vej-dovsky, op. cit. infra, p. 162). In Typhaeus from India it is apparently absent, and the mouth therefore terminal.
In Microchaeta Rappi the somites are secondarily annulated, as in the Leeches. The number of annuli in the first and in each somite from the ninth onwards is three: but in the second to the seventh somite inclusive it is either six or seven.
The cuticle is thin, transparent, and variable in thickness in different regions of the body. It is said to consist of two layers, an outer longitudinal and a thicker inner circular fibrillar layer. It is pierced by minute pores, the orifices of unicellular hypodermic glands.
The hypodermis or epidermis consists of a single layer of cells. In the intersegmental furrows they are shorter than elsewhere, and are non-glandular: whereas in other regions glandular and non-glandular cells occur intermixed. The non-glandular cells are either cylindrical with several basal processes, or more or less globular with a slender external process and one or more basal processes. The latter kind of cell lies at a deeper level than the former. The glandular cells vary somewhat in shape and in the nature of their contents, probably in accordance with their state of activity. According to Professor Ray Lankester, processes of pigment cells belonging to the subjacent connective tissue pass up, especially in Z. o/idus, between the hypodermis cells, as in the Leech. In the clitellum non-glandular cells are not to be found in the common Earthworm and some of its allies; in others they are present in reduced numbers. Its glandular cells are divisible into a more superficial layer with coarsely granular contents which stain readily with carmine, and a deeper layer with finely granular contents which do not stain with the same reagent. In Allolobophora (Dendrobaena) rubida, Vejdowsky traced nerve-filaments into apparent continuity with the glands.
Capillaries penetrate between the clitellar glands, and are very numerous in the common Earthworm: in some species they are few in number. In Perionyx and Megascolex the general hypodermis is vascularised as in the Leech. An elastic basement membrane separates the hypodermis from the muscular layers of the body-wall. It is highly developed in Perionyx and Perichaeta.
The muscles are disposed in an outer circular and an inner longitudinal layer. Each muscle-fibre corresponds to a single cell with a well-developed cortical fibrillar substance and a central medulla or protoplasm which is scanty, and in which a nucleus has been found in L. olidus and Phreoryctes, two worms in which the fibrillar cortex is sometimes deficient at one spot, so that the muscle-cell becomes coeomyarian. In L. terrestris and some other Lumbrici and a few other Earthworms, as well as in Serpula and Protula among Polychaeta, the simple layer of longitudinal cells found in the lower Oligochaeta is disposed in parallel folds, the bi-pinnate bundles of Claparede, which are held together by intervening connective tissue. The muscle-cells of the longitudinal coat in other Chaetopoda are arranged in more or less regular groups or bundles. The longitudinal coat is divided in Oligochaeta into distinct tracts by the projection internally of the sacs of the setae. The bundles of protrusor or parieto-vaginal muscles attached to the bases of the sacs in question are derived however from the circular coat. Bundles of muscular or inter-follicular fibres pass from the bases of the sacs in the dorsal row to the bases of the sacs in the ventral row. They appear to be special formations.