Studies on Earthworms. - Journal of Cell Science

2M WILLIAM BLAXLAND BENHAM.
In Echinodrilus we have still four groups, but there are
five setee to each group.
In Hypogseon, Savigny described nine equidistant setae, of
which one was said to be in the mid dorsal line.
In Tritogenia, Kinberg, only six setse are present in each
somite. The number is greatly increased in Perichseta,
•where there may be as many as 100 to the somite, and in
Periony x there are thirty. These in each case are equidistant
and form a complete ring round each somite.
In Megascolex, again, this arrangement is varied by an
interruption in the ring in the mid dorsal line, whilst in Beddard's
Pleurochseta, as well as this dorsal break, there is a
similar break ventrally.
The setse are not always simply pointed, as Clapar&de supposed,
but in many cases are variously ornamented, as in Rhinodrilus,
throughout the body; in the genital setse of Urochaata
and of AJcanthodrilus; whilst in Urochseta all the
setse are bifid at their free extremity. Moreover they are
modified in certain parts of the body, as Hering (5) has shown,
for copulation: e. g. in Lumbricus, on clitellum, and on the
somites xv and xxvi.
Pores.—Perrier has pointed out a sort of relationship between
the nephridiopores and the couples of setse; in some genera
these pores are in front of and slightly dorsad of the ventral
couples (setse 1 and 2), as in Lumbricus, Titanus, Pontodrilus,
whilst in other genera the pores have a similar relation
to the lateral couples (3 and 4), Ehinodrilus, Eudrilus,
Acanthodrilus, AnteuSj Moniligaster [also in Microchseta].
Plutellus is exceptional at present in showing an alternation
of the nephridiopores with the seta 2 and with seta 4 in
consecutive somites, whilst the first few pairs are in line with
the third seta. Again, they are, in Urochseta, related to the
third seta throughout the body. They remain in this line even
when the setse alternate, though one would expect, if there
were any relation between them and the setse, that the nephridiopores
would also alternate.