The mouth and anus are usually ventral; but in _M. glabrum_ the anus is dorsal, and in a few species both apertures are carried on to the back by the great development of the ventral surface. The alimentary canal is provided with a protrusible pharynx; the intestine is branched; amongst its branches is the coelom, packed with eggs, and functioning as a uterus (usually called "ovary"). The true ovary is a small mass of cells on each side, a proliferation of the coelomic epithelium covering the intestinal wall. A median continuation of the {344}uterus passes backwards above the rectum, and opens either into it or by an independent pore dorsal to the anus. The "lateral oviducts" of Nansen are nephridia with ciliated funnels opening into the coelom (uterus), and with pores leading into the cloaca on its ventral surface; or, in _M. belli_ Wheeler, opening to the exterior. The two testes are branched, and each sperm-duct opens laterally on a papilla, just outside the third parapodium of each side. Wheeler[399] has recently shown that in the young _Myzostoma_ the spermatozoa ripen before the ova, so that it is functionally a male; before the spermatozoa are all discharged the ova mature, and the animal is for a time hermaphrodite; later on, however, when all the spermatozoa are used up, the worm is a female. Beard's "dwarf males" are therefore merely the young of hermaphrodite forms. In cysticolous species each cyst usually contains a large female individual and a small male. In these cases the young one (male) discharges all its spermatozoa before the ova ripen, so that a period of immaturity intervenes and a true hermaphrodite condition is omitted; the animal is at first male, and later female. The Myzostomaria are thus "protandric hermaphrodites." The affinity of these animals has been much discussed; they superficially resemble the Tardigrada in many anatomical features, and differ greatly from Chaetopoda, but as they possess the characteristic chaetae or parapodia, and pass through a larval stage[400] similar to that of the Polychaetes, there is no doubt that they are closely allied to the group, and indeed may be regarded as degenerate Chaetopods. It has been suggested that they form a passage group between them and the Tardigrada; and von Graff forms a group Stelechopoda, to include the Myzostomaria, the Tardigrada, and the Linguatulida. OLIGOCHAETA (EARTHWORMS, ETC.) AND HIRUDINEA (LEECHES) BY F. E. BEDDARD, M.A. (Oxon.), F.R.S. Prosector to the Zoological Society {347}CHAPTER XIII OLIGOCHAETA (EARTHWORMS AND THEIR ALLIES) INTRODUCTION—ANATOMY—REPRODUCTION—BIONOMICS—DISTRIBUTION—CLASSIFICATION— MICRODRILI AND MEGADRILI