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" {349}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