Structure of the Body-Wall.—The body-wall consists in all Oligochaeta of three recognisable sheets of tissue. Outside is the epidermis, which always consists of a single layer of cells, except in the clitellar region of earthworms. It is a point of difference between the aquatic genera and the terrestrial forms that in the former the clitellum is only one cell thick, while in the higher Oligochaeta it is made up of more than one layer of cells. The epidermis is ciliated only in the genus Aeolosoma, and there only on the prostomium. It secretes a thin layer of chitin, which is defective opposite to the glandular cells, and becomes therefore perforated by numerous pores. The structure of the epidermis of Lumbricus has been studied by Cerfontaine, whose recent account[[407]] of the same is the fullest and most accurate that exists.
Underneath the epidermis comes a layer of circular muscle-fibres, and underneath this again a layer of longitudinal muscles. In both layers the fibres have a softer core, outside which lies the radially striated muscular substance. The fibres are embedded in a granular matrix. It used to be considered at one time that such medullated fibres were distinctive of leeches as opposed to Oligochaeta. Their existence has been really known in the Oligochaeta since the researches of Ratzel; but Cerfontaine has fully described them, and emphasised the fact that the fibres of both circular and longitudinal coats are alike in this respect.
Fig. 187.—Chaetae of Oligochaeta. × 10. (After Michaelsen, Stolc, and Vejdovsky). 1, 2, Penial chaetae of Acanthodrilus georgianus; 3, Spirosperma; 4, Ilyodrilus; 5, Lophochaeta; 6, Tubifex; 7, 8, Nais; 9, Bohemilla. Figs. 3-9 are ordinary chaetae.
Chaetae.—The passive organs of locomotion in these animals are the chaetae, which are absent in only one family, Discodrilidae, and in one other genus, Anachaeta. In this latter worm the chaetae are represented by large glandular cells, which seem to correspond to the cells from which the chaetae arise in other forms. They are in this case, as in the others, cells of the epidermis. The chaetae of the Oligochaeta are not quite so variable in form as in the marine Polychaeta (see Fig. 138, p. [267]). Figs. 187 and 188 illustrate some of the principal shapes which these bristles assume. The most prevalent form is an elongated S, which has been aptly compared to the mathematical sign ∫. This kind of chaeta is found in all earthworms, and in not a few aquatic genera such as the Lumbriculidae. In some of the latter and in the Tubificidae and Naids there is the same form of chaeta, which is cleft at the free end, and possibly enables the worm to grasp the leaves of aquatic plants, and otherwise facilitates progression in a laxer medium than the stiff soil frequented by the earthworms. Even earthworms, at any rate the genus Pontoscolex, have chaetae of this kind; some of the aquatic Oligochaeta have elongated and hair-like bristles, such as that of Tubifex.
Fig. 188.—Chaetae. × 10. 1, Onychochaeta; 2, Pontoscolex; 3, Trichochaeta; 3b, the same, more highly magnified.
In the Tubificid Lophochaeta (Fig. 187, 5) the chaetae are ornamented on both sides with delicate processes, which give them the appearance of Crustacean hairs. Among earthworms the simple S-like form is sometimes complicated by the development of sinuous ridges upon the distal end. No doubt these bristles enable their possessor to get a firmer grip of adjacent objects; they are very commonly found, in the family Geoscolicidae, upon the segments of the clitellum, and permit of a firmer union during sexual congress. In no Oligochaeta are the chaetae borne upon parapodia, as is the case with the Polychaeta; but in many of the aquatic forms there are a considerable number to each bundle. In earthworms the number of chaetae varies greatly. The common earthworms of this country, belonging to the genera Lumbricus, Allolobophora, and Allurus, have only eight chaetae upon each segment of the body, and these are then, as a rule, arranged in pairs or rather couples, two of each on each side of the body. The genus Perichaeta and some of its allies have a much larger number of chaetae to each segment, disposed in a continuous row round the middle of the segment. The intermediate condition is to be seen in the genus Deinodrilus, where there are twelve in each segment, and in certain members of the genus Megascolex, where there are eight in each segment in the anterior region of the body, the number increasing in the posterior segments. The four bundles of chaetae in the Naids and Tubificids have been likened to the notopodia and neuropodia of the Polychaetes; but it does not seem certain that this comparison is justifiable. It was at one time thought that the continuous circle of chaetae of the Perichaetidae was the primitive condition; but Professor Bourne has lately found that in Perichaeta the young embryos have not got this continuous circle; it is only acquired later.
Branchiae.—The Oligochaeta were called by Cuvier the "Annélides abranches sétigères." But the epithet "abranches" is now known to be inaccurate. In fact it really was so when Cuvier wrote; for naturalists were at that time well acquainted, chiefly through the elaborate work of O. F. Müller, with the little fresh-water Naid Dero, the posterior extremity of which is provided with a varying number of branchial processes. These are furnished with looped blood-vessels and are covered externally by cilia, so that the water containing oxygen is constantly renovated. The second instance of a gilled Oligochaete was discovered in the very same family. Professor Bourne[[408]] of Madras found in "tanks" a Naid which he named Chaetobranchus, in which the head segments, to the number of fifty or so, are provided with long ciliated processes, which as a rule enclose the dorsal chaetae of their segments, and in addition a capillary loop. Curiously enough, this very same worm made its appearance in the Victoria regia tank at the Botanical Gardens in the Regent's Park, whither it had in all probability been accidentally imported. Two members of the family Tubificidae were the next examples of gilled Oligochaeta made known to science; one of these, Branchiura sowerbyi,[[409]] appeared also in the Botanical Gardens, so that its native home is unknown. It differs from Chaetobranchus in that the gills are at the posterior end of the body, and are contractile; during the life of the worm they are in continual motion. A species of the South American genus Hesperodrilus,[[410]] H. branchiatus, is also gilled, and, so far as can be made out from a spirit-preserved specimen, the gills are precisely of the same pattern and contractility as those of its ally Branchiura. Possibly Branchiura ought to be included in the same genus with Hesperodrilus. A worm which was originally described by Grube as Alma nilotica, should really have been placed before the three last-mentioned instances; but as this worm was only known from a fragment, and as the description was not by any means full, it was not thoroughly believed in; it was surmised that it might be a member of some marine genus, perhaps of the Capitellidae. Oddly enough, the same worm was independently described by a different name, Digitibranchus niloticus, a few years later by Levinsen. Quite recently Michaelsen has found by a reference to the original types that this worm is really gilled, and that it is specifically identical with a worm which had been given a totally different name, viz. Siphonogaster. The fact that the gills of the latter had been overlooked was readily explained by the circumstance that they are retractile, and not merely contractile. But all the species of the genus Siphonogaster, or Alma, as it ought really, following the rules of priority, to be called, have not got gills, as is the case too with the genus Hesperodrilus. The gills of Alma are branched, and there is therefore no longer any justification whatever for defining the Oligochaeta as a group of Annelids without gills. The simple gill-like processes of Chaetobranchus might have been held to be not accurately comparable to the more complex structures which we find in the marine worms.
