BRITISH ANNELIDS.
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sent only one species of Allurus (subject to variation, however, as we
shall see later on), it will be utterly impossible to confound it with
Allolobophera when we have once seen and examined it. We are
thus narrowed down practically to the two genera Lumbricus and
Allolobophora, and have, as external guides to their distinction, the
mode of insertion between lip and peristomium, the colour, and the
setae. Since the latter are variable we may be obliged occasionally
to resort to anatomy before we can be absolutely certain about a
given species; but I believe that I shall be able to show as we pro-
ceed that all the species may be readily distinguished by external
characters alone, if only they are mature. I shall endeavour to give
such unmistakable clues to the identification of each species by
external means as shall render the use of the knife and the micro-
scope unnecessary.
Those who have followed me thus far will have learned which
are the leading portions of the body of a worm, and what parts must
be particularly observed in order to obtain a clue to their identity.
I may add now a few more details which will be helpful, and explain
some terms which will be constantly met with in the study of
Annelid literature.
Beginning with the front or anterior portion of the body we find
that there is no distinct head, while no organs of vision or hearing
are anywhere apparent. There is a retractile lip, usually called the
prostomium on account of its being in front of and above the mouth
(stoma). The first ring, segment, or somite, bears no setae or
bristles, and is called the peristomium, because it surrounds the
mouth. Some works include this ring in all calculations relating to
the number of segments, but it is usual in England to omit it, and
begin to reckon from the first segment which carries bristles. The
setae are organs of locomotion. Along the back, in the groove between
each segment, one will be able to discover a series of pores or
punctures which look as though a pin had been thrust through the
skin. These are the dorsal pores of which an account will be found
in "Science Gossip," December, 1891. The male pore, found on
segment fifteen, except in the case of Allurus, is to be looked
for on the lower surface of the body. In some species the opening
is seated on a cushion or papilla of very delicate structure, which
gives it great prominence ; but in other cases only a well-trained eye
will detect it. The Greenworm and the Common Earthworm have
the most prominent papilla for the male pore, and these should be