A very remarkable genus, Brachydrilus, has lately been described by Dr. Benham,[[413]] in which each segment has two pairs of nephridia instead of a single pair. More recently, certain Australian forms, which I propose to unite on this account into a genus Trinephrus, have been discovered which have no less than three distinct and separate pairs in each segment.[[414]]
Fig. 191.—Section through body-wall of Megascolides australis, highly magnified. (After Spencer.) 1, 4, 5, 6, Coils of nephridia; 2, funnel; 3, septum; 7, external apertures.
In many Megascolicidae there is a nephridial system of a different character. In Perichaeta when dissected the nephridia appear, on account of their minute size, to be altogether absent. There is, however, in most Perichaetidae, in many Acanthodrilidae, and in many Cryptodrilidae a mass of minute tubules which cover the inside of the body-wall, and open on to the exterior by innumerable openings; there may be in a single segment one hundred or more of these external orifices, which are scattered about irregularly. It is at present uncertain whether these minute tubes are connected among themselves, thus forming a network passing through the septum and from segment to segment, or whether each tube is isolated from its fellows, and forms a distinct nephridium, of which there are many in each segment and entirely separate. This is, however, certain, that the complex nephridial systems of at any rate Octochaetus and Megascolides are derived from the multiplication of a single pair of tubes which are alone present in the embryo. In Perichaeta the minute nephridia are furnished with coelomic funnels; in Octochaetus they are not, except in the case of certain nephridia which open into the terminal section of the intestine.
Both at the anterior and at the posterior end the nephridia occasionally open into the alimentary canal. In various genera the first pair of nephridia are larger than the others, and open into the buccal cavity; it seems likely that they serve as salivary glands. A somewhat similar condition of things exists in Peripatus (vol. v, p. 17). In Octochaetus multiporus, for example, there is a large tuft of nephridial tubes in the anterior region of the body, which opens by a long muscular duct into the buccal cavity. In the same species a good many of the nephridial tubes open into the posterior section of the intestine, reminding one of the anal vesicles of the Gephyrea (p. [436]) and of the Malpighian tubes of the Arthropods.
In many Eudrilidae the ducts of the paired nephridia form a network in the body-wall, which opens on to the exterior by many pores.
Alimentary Canal.—The digestive tube is perfectly straight in nearly all Oligochaeta. Only in Plagiochaeta and a species of Digaster is it twisted in the intestinal region in a corkscrew-like fashion. The mouth is under the buccal lobe (where, as in the majority of cases, this is present); the anus is mostly terminal, or rarely, e.g. Criodrilus, a little in advance of the end of the body on the ventral side. In the simpler forms three regions can be distinguished, which are themselves simple in structure. The mouth leads into a buccal cavity, which in its turn opens into the pharynx; the latter is muscular, with thick walls. The narrower oesophagus opens into the wider intestine, which opens posteriorly, as already stated. In the earthworms there is as a rule some complication. The oesophagus bears certain glandular appendages, the calciferous glands; and a part of it is modified into a gizzard. The gizzard is merely a portion of the oesophagus with very much thickened muscular walls and with a stout lining of chitin. It is not universally present among earthworms, and when present varies much in position. The rule is that one gizzard only is present. In Digaster, as is implied by the name, and in some other forms there are two in successive segments; in Trigaster, as the name also indicates, there are three gizzards; in Moniligaster and the Eudrilids Hyperiodrilus and Heliodrilus there are four to six; and a few other forms also have a considerable number of gizzards. The calciferous glands are diverticula of the oesophagus with folded and sometimes ciliated walls; their epithelium secretes calcareous particles, which are frequently of crystalline form. Darwin supposed that this secretion was provided in order to negative the humus-acids of the soil which is the food of earthworms. These organs are usually paired, but in the Eudrilidae there are unpaired as well as paired glands; the unpaired calciferous glands lie ventrally. These glands are totally wanting among the aquatic families, with the sole exception of the Enchytraeidae. In a few of these there are either paired or single glands of a very similar nature; Dr. Michaelsen has suggested that the function of these is rather absorptive than secretory. From the median unpaired gland of Buchholzia arises the dorsal vessel, which at first forms a sinus round the glandular epithelium; the epithelium, like that of the nephridia, is perforated by the ducts. In certain Oligochaeta there are some curious modifications of the calciferous glands. In Stuhlmannia and a few other Eudrilidae the oesophagus is beset with a larger number of paired structures than in any other genera of the family, where the calciferous glands are more limited in number. These glands consist of a short tube lined with epithelium opening into the oesophagus. Round this is a mass of cellular tissue, but the outlines of the constituent cells are lost; the whole is permeated with abundant blood-vessels. This layer seems to be peritoneal, and the entire gland seems to have lost its function as a secretory organ, and to have taken on some function in connexion with the vascular system. An analogous modification is to be found among the Enchytraeidae. In certain forms there is a structure known as the cardiac body; this is a chord of cells lying in the dorsal blood-vessel at the point where it springs from the intestine. It is tempting to regard this cellular rod as being the altered dorsal glandular pouch already spoken of, which is surrounded by a blood sinus.
Reproductive Organs.—All the Oligochaeta are hermaphrodite animals. But, as is the case with other hermaphrodites, the male and female organs are in many cases mature at different times, thus leading to a practical unisexuality. Many of the aquatic forms appear to have fixed times for breeding, which may be in the winter or in the summer; but the earthworms are as a rule sexually mature the whole year round. Various accessory organs are developed in the majority of cases. In all, the reproductive glands lie in successive segments and are attached to the septa, from the peritoneal covering of which they originate. Their actual position differs greatly in different genera; the position is constant only in the earthworms, where the testes are in the tenth and eleventh segments and the ovaries in the thirteenth, in exactly corresponding situations. A few earthworms have only one pair of testes. The only exception, among terrestrial forms, to the position of the generative organs is in the family Moniligastridae, which show so many other affinities to the lower forms of Oligochaeta. In this family the ovaries have moved one or two segments forward. Among the fresh-water families the position of the testes and of the ovaries is not so uniform. They are generally more anterior than in the terrestrial genera, particularly the ovaries.
One of the chief differences between the Oligochaeta and the Polychaeta is that the reproductive organs of the former have special ducts to convey their products to the exterior. In Aeolosoma, the only exception to this rule, Dr. Stolc[[415]] has shown some reasons for believing that certain nephridia, but slightly altered in form, serve as the conduits of the spermatozoa, whilst the ova are extruded through a pore upon the ventral surface of the body. In the Enchytraeidae the same pore for the extrusion of the ova appears to exist; but a nearer examination shows that it is really not a mere perforation of the integument, like the dorsal pores, for example, but that its internal orifice is fringed with cells which seem to represent a rudimentary oviduct; perhaps Aeolosoma typifies a last stage in the reduction. Even so high in the scale as in the genus Nemertodrilus (Eudrilidae), there is an oviduct which can only be compared with that of the Enchytraeidae. Elsewhere the oviducts are a pair of tubes with a wide, funnel-shaped, and ciliated mouth, which leads to the exterior by way of a ciliated tube of varying length.
The sperm-ducts are of an essentially similar structure; but they are commonly much longer, passing through a variable number of segments on their way to the exterior. In most earthworms there are, moreover, two of them on each side instead of only a single pair, as is the case with the oviducts. Among the Tubificidae, Naids, and other aquatic families there are only two sperm-ducts, one on each side of the body. But this is not a character of the aquatic families, for the Lumbriculidae have generally two pairs, as in the earthworms. It is, however, a rule with hardly an exception, that among the aquatic Oligochaets the sperm-ducts open, as do the oviducts in all Oligochaets, upon the segments following that which bears internally the ciliated funnel. It is only in the Moniligastridae among earthworms that the sperm-duct only traverses two segments in its course. But where it is short as regards the actual distance traversed between the two extremities, the tube itself is commonly long and coiled.