dellodrilus_ and _Astacobdella_ have none. The vascular system is as
in the lower Oligochaeta. There are two chitinous jaws in the buccal
cavity, a dorsal and a ventral, which are of specially complicated
structure in _Cirrodrilus_.

LITERATURE.--F.E. Beddard, _A Monograph of the Oligochaeta_ (Oxford,
1895), also _Quart. Journ. Micr. Sci._, 1886-1895, and _Proc. Zool.
Soc._, 1885-1906; W.B. Benham, _Quart. Journ. Micr. Sci._, 1886-1905;
W. Michaelsen, "Oligochaeta" in _Das Tierreich_, 1900, and _Mitth.
Mus._ (Hamburg, 1890-1906); A.G. Bourne, _Quart. Journ. Micr. Sci._,
1894; H.J. Moore, _Journ. Morph._, 1895; F. Vezhdovsky, _System d.
Oligochaeten_ (Prague, 1884), and _Entwicklungsgeschichtliche
Untersuchungen_; and numerous papers by the above and by G. Eisen, E.
Perrier, D. Rosa, R. Horst, L. Cognetti, U. Pierantoni, W. Baldwin
Spencer, H. Ude, &c., and embryological memoirs by R.S. Bergh, E.B.
Wilson, N. Kleinenberg, &c.

HIRUDINEA.--The leeches are more particularly to be compared with the
Oligochaeta, and the following definition embraces the main features in
which they agree and disagree with that group. Setae are only present in
the genus _Acanthobdella_. Eyes are present, but hardly so complex as in
certain genera of Polychaetes. The appendages of the body are reduced to
branchiae, present in certain forms. A clitellum is present. The
segments of body are few (not more than thirty-four) and fixed in
number. The anus is dorsal. One or two (anterior and posterior) suckers
always present. Nervous system always in coelom. Coelom generally
reduced to a system of tubes, sometimes communicating with vascular
system; in _Acanthobdella_ and _Ozobranchus_ a series of metamerically
arranged chambers as in Oligochaeta. Nephridia always paired, rarely
(_Pontobdella_) forming a network communicating from segment to segment;
lumen of nephridia always intracellular, funnels pervious or impervious.
Alimentary canal sometimes with protrusible proboscis; never with
gizzard or oesophageal glands; intestine with caeca as a rule. Jaws
often present. Testes several pairs, rarely one pair, continuous with
sperm ducts; ovaries, one pair, continuous with oviducts; generative
pores single and median. No separate spermathecae or septal chambers for
the development of the ova and sperm. Eggs deposited in a cocoon.
Development direct. No asexual generation. Fresh-water, marine and
terrestrial. Parasitic or carni