II. Annelida Oligochaeta
Reproductive System. Replacement of Lost Parts. Parasites. Symbiosis. Distribution. A Note on Classification.
The Oligochaeta are hermaphrodite, the sexual elements being developed in certain anterior segments in the form of testicles, deferent canals, receptacles, copulatory papillae in some males, and ovaries and accessories in the female. In each segmet, with a few exceptions (while there are two in each in Lumbricus), the segmental organs occur. Claparede thought that in the Limicola the latter are absent from the segments bearing the oviducts and seminal receptacles, but present in those having male organs; while in Lumbricus they are present in all the segments except the four last. In some, special glands exist for secreting the egg-capsules. A cincture or clrellum occurs in certain segments anteriorly (in Lumbricus from the eighteenth to the twenty-ninth or thirtieth, and in the others from the tenth to the fifteenth segment). The ova are deposited in chitinous capsules (containing one or more), and the young issue there from in a tolerably complete condition. Besides the ordinary development by ova, it has long been known that Nais and Choetogaster exhibit fissiparous reproduction. In Nais, after a certain degree of growth has been reached, budding takes place, so that several tolerably complete young forms may be found attached to the adult. The process goes on until the Nais has been reduced to twelve or fourteen rings; then a pause occurs, and the animal increases in length to forty or fifty rings, when a new cycle commences by division in the middle of the body. Choetogaster shows similar features, the budding taking place between the third and fourth segments in the androgynous form -- manyzoids or buds being attached in line; but while the second in position is one of the newest, the second in age is near the middle of the series.
Reproduction of lost parts readily takes place in this group. Parasites, such as Gregarinoe, Opalinoe, Nematoids, and larval cestodes, are met with internally, and Vorticelloe are common externally. Commensalism is seen in Choetogaster, which lives on the pond-snails (Limnoeus and Planorbis), and Stylaria is stated also to be ectoparasitic.
They are distributed over the land and fresh waters (in sand an dmud) of the whole world: very few are marine.
Two of the most important classifications are those of D'Udekem and Clalarede. The former arranges the suborders according to the gemmiparous, or non-gemmiparous condition of the constituents. The latter group live in earth or mud, are unable to swim or follow their prey, and have always their organs of generation. The former are elegant little worms, living in stagnant or running water, able to swim, and their generative organs are developed only at certain periods. Claparede divides them into two families, viz., the O. Terricola (including the earthworm and its allies), and the O. Limicola, an arrangement much resembling Grube's Lumbricina and Naidea.
Read the rest of this article:
Annelida - Table of Contents