BY
F. E. BEDDARD, M.A. (Oxon.), F.R.S.
Prosector to the Zoological Society
CHAPTER XIII
OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES)
INTRODUCTION—ANATOMY—REPRODUCTION—BIONOMICS—DISTRIBUTION—CLASSIFICATION—MICRODRILI AND MEGADRILI
The Oligochaeta form a well-marked branch of that exceedingly large assortment of animals vaguely spoken of as worms, and embracing a number of types many of which have no near relationship to each other. From this great and unnatural group, which has survived as "Vermes" even in some quite modern text-books, we can separate off those forms which show a plain segmentation or division of the body internally as well as externally into a series of more or less similar rings, as a Class Chaetopoda. This Class, consisting of the Orders mentioned on p. [241], includes the worms which form the subject of the present chapter—the Oligochaeta, as they were originally called by Grube, on account of the fewness of their chaetae as compared with the number possessed by the majority of the Polychaeta.
Our knowledge of this group, as of so many others, dates from Aristotle, who called the earthworms the "intestines of the earth." But it is only very recently that the numerous and remarkable genera of exotic earthworms have been anatomically investigated; indeed the common British species was not really well known before the publication of the memoirs of Lankester[[401]] and Claparède[[402]] in 1864 and 1868, in spite of the elaborate quarto devoted to it by Morren,[[403]] the botanist, in 1826. Some of the aquatic species afforded material to Bonnet and Spallanzani for their experiments upon the powers of regeneration of the animals when cut into fragments, while the work of O. F. Müller[[404]] upon various Naids is a monument of careful anatomical description. Our knowledge of the aquatic Oligochaeta does not appear to have advanced so rapidly as has that of the earthworms.
External Characters.—The most salient external characteristic of this group of worms, which vary from 1 mm. to 2 metres in length, is of course the segmentation. The entire body is divided into a number of rings, which are for the most part similar to each other; a fragment of an earthworm's body could not be accurately replaced unless it had been cut from the anterior region. There is precisely the same regular segmentation in the aquatic representatives of the Order. At the anterior end of the body in the common earthworm (and in nearly all Oligochaeta) is a small unpaired lobe, which overhangs the mouth, and is usually termed the prostomium; the mouth itself is surrounded by the first segment of the body, which never bears any chaetae in any Oligochaete. The prostomium is occasionally greatly developed, and in such cases doubtless forms a tactile organ of importance. This is especially the case with the South American genus Rhinodrilus, where the lengthy prostomium can be retracted at will. The aquatic Nais lacustris (= Stylaria proboscidea) has also an exceedingly long prostomium, which cannot, however, be retracted, though it is contractile. At a certain distance from the anterior end of the body, fixed for the species, but varying greatly from genus to genus and from species to species, is the clitellum. This region of the body (popularly believed to mark the spot where a worm divided by the gardener's spade has come together again) is associated with the reproductive function, and serves to secrete the cocoon in which the creature's eggs are deposited. It has in the earthworm a thick glandular appearance. A more minute examination of the worm's body will show the orifices of the reproductive ducts and of the excretory organs which will be found described below. In addition to these, all British earthworms and a large percentage of the tropical forms have a row of pores along the back, which are between the successive segments in the median line. These "dorsal pores" open directly into the body-cavity, and are mere perforations of the body-wall, not tubes lined by a special layer of cells. Professor Spencer of Melbourne[[405]] has observed a giant earthworm (Megascolides australis) of Gippsland which, when held in the hand, spurts out, to a height of several inches, the fluid of the body-cavity through its dorsal pores. The burrows, he remarks, are coated over with the same fluid, which is regarded by him as a lubricant. This, however, considering that the glandular cells of the epidermis can secrete a mucous fluid, seems to be an expensive use to which to put the important fluids of the interior of the body. It is more probable that the dorsal pores are a means of getting rid of waste products. Lim Boon Keng[[406]] suggests that the coelomic fluid possesses a bactericidal function. The dorsal pores are missing in many earthworms, and without exception in those Oligochaeta which live in water; but these latter worms have a pore upon the head, which appears to be wanting in the earthworms. Dr. Michaelsen has thought that the head-pore serves to relieve the brain from undue watery pressure—to act, in fact, as a kind of safety-valve for the liberation of superfluous fluid.
In some foreign worms the pores of the reproductive ducts are conspicuous external features (Fig. 197); even in our British species the turgescent male apertures upon the fifteenth segment are sometimes quite obvious.