groups. In both, the
nephridia are all alike; there are no jaws; the prostomium rarely has
processes. The body is often divisible into regions.

LITERATURE.--W.B. Benham, "Polychaeta" in _Cambridge Natural History_;
E. Claparede, _Annelides chetopodes du golfe de Naples_ (1868 and
1870); E. Ehlers, _Die Borstenwurmer_ (1868); H. Eisig, _Die
Capitelliden_ (Naples Monographs), and development of do. in _Mitth.
d. zool. Stat. Neapel_ (1898); W.C. M'Intosh, _"Challenger" Reports_
(1885); E.R. Lankester, Introductory Chapter in _A Treatise on
Zoology_; E.S. Goodrich, _Quart. Journ. Mic. Sci._ (1897-1900); E.
Meyer, _Mitth. d. zool. Stat. Neapel_ (1887, 1888), as well as
numerous other memoirs by the above and by J.T. Cunningham, de St
Joseph, A. Malaquin, A. Agassiz, A.T. Watson, Malmgren, Bobretsky and
A.F. Marion, E.A. Andrews, L.C. Cosmovici, R. Horst, W. Michaelsen, G.
Gilson, F. Buchanan, H. Levinsen, Joyeux-Laffuie, F.W. Gamble, &c.

OLIGOCHAETA.--As contrasted with the other subdivisions of the
Chaetopoda, the Oligochaeta may be thus defined. Setae very rarely
absent (genus _Achaeta_) and as a rule not so large or so numerous in
each segment as in the Polychaeta, and different in shape. Eyes rarely
present and then rudimentary. Prostomium generally small, sometimes
prolonged, but never bearing tentacles or processes. Appendages of body
reduced to branchiae, present only in four species, and to the ventral
copulatory appendages of _Alma_ and _Criodrilus_. Clitellum always
present, extending over two (many limicolous forms) to forty-five
segments (_Alma_). Segments of body numerous and not distinctive of
species, being irregular and not fixed in numbers. In terrestrial forms
dorsal pores are usually present; in aquatic forms a head pore only.
Anus nearly always terminal, rarely dorsal, at a little distance from
end of body. Suckers absent. Nervous system rarely (_Aeolosoma_) in
continuity with epidermis. Vascular system always present, forming a
closed system, more complicated in the larger forms than in the aquatic
genera. Several specially large contractile trunks in the anterior
segments uniting the dorsal and ventral vessels. Nephridia generally
paired, often very numerous in each segment, in the form of long,
much-coiled tubes with intracellular lumen. Gonads limited in number of
pairs, testes and ovaries always present in the same individual. Special
sacs developed from the intersegme