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 {361}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.