THE EXTERNAL FEATURES OF EARTHWORMS AND THEIR RELATION TO HABIT AND
ENVIRONMENT
To the very inexperienced eye all earthworms might appear to be
quite similar in detail as they undoubtedly are in general form.
But it needs not a great deal of examination to detect even salient
characteristics whereby one kind may be distinguished from another;
to the expert it is possible in very many cases to go no further
than the outside before assigning its correct place in the system
to a given example. The general external features of this group
of worms have been already dealt with in another chapter. To some
of these we again direct attention in a more elaborate fashion in
order to emphasise the possible meanings of the variations met with
apart from their use in systematic arrangement. It is difficult to
say in comparing one worm with another what is the most salient
external difference. There are however a few which may be regarded
as equally conspicuous on a nearer examination of the specimens.
The varying position and greater or less extent of the clitellum,
the longer or shorter retractile or nonretractile prostomium, the
position of the usually conspicuous male pores, and the existence
of in the first place and--when present--the numbers and situation
of the so-called genital papillae are among the most obvious. The
setae and their position we treat of under the heading of the
modification of the worms to lead a terrestrial life; and though
these chitinous organs differ greatly they do not concern us in
the present section. The girdle or clitellum ('eminentia quasi
ulcerata') has been long observed as a character of these animals
and it is one which distinguishes them from all other worms except
the leeches and a very few marine Polychaeta. This modified region
of the body is often of a different colour to the rest and has a
glandular look which readily enables one to recognise its position
and limits, though its obviousness is less in some cases. It either
forms a complete ring round the body or is developed upon the dorsal
surface and only to a slight extent upon the ventral surface. Its
use, as is well known, is to secrete the cocoon in which the eggs are
deposited; and the epidermis which forms it is thickened and more
glandular than that in other regions of the body. Among earthworms
it is doubtful whether the clitellum ever occupies less than three
segments; it consists of three only in the great majority of species
of the marked genus _Pheretima_. From this lowest level it extends
in other forms, and in the partially aquatic African genus _Alma_
it may occupy as many as forty segments. The position also varies
from genus to genus and from species to species. It is sometimes
further forward and sometimes further back. In the remarkable family
Moniligastridae this organ is developed earlier in the body than in
any other group of true earthworms, consisting of four segments or
so commencing with the tenth. As a rule the clitellum begins further
back than this--the thirteenth or fourteenth being a common place for
the first commencement of the organ among the Megascolecidae, while
among the Geoscolecidae and Lumbricidae it is generally much further
back, commencing in _Alma_ at the forty-fifth. These details might
be increased to many pages; but enough has been said to emphasise
the variability of the organ. What reason can be assigned to this
variability, which might be supposed unnecessary in view of its
functions? There are perhaps two suggestions that may be made, though
many facts are lacking which might offer confirmation or refutation
of either of these. It is to be noticed that on the whole the older
types such as the Moniligastridae and the Megascolecidae (including
for this purpose the Eudrilidae) have clitella which are short. There
are a few but not many exceptions. These older types do not seem
capable of extending their range with any rapidity. It is true that
here again there are exceptions, notably many species of _Pheretima_
which are considered under the section which deals with the
migration of these animals. On the other hand the Lumbricidae have on
the whole a more extensive clitellum and so have many Geoscolecidae.
It is obvious that of all earthworms the Lumbricidae is the family
which has the greatest capacity of migration and adaptation to new
circumstances. The reason for this may be that in the latter case
the more extensive clitellum produces a larger cocoon which in
its turn can hold and cherish while they reach maturity a larger
number of embryos. Much remains to be learnt under this heading. But
the comparatively small clitellum of the large Ceylon _Megascolex
coeruleus_ only contains two embryos, while the also comparatively
small cocoon of the large and restricted _Octochaetus multiporus_
(limited to the South Island of New Zealand) only contains a single
embryo. This latter fact may be regarded as fairly well established
since I myself examined quite fifty cocoons.
On the other hand larger numbers seem to arrive at maturity in the
cocoons of _Allolobophora_. The more extensive clitellum must produce
a relatively larger cocoon, and it is interesting to note that the
cocoon of the widely distributed genus _Criodrilus_ (Europe and South
America) is very long although not of great diameter. However the
facts are not sufficiently great to dogmatise much upon this subject.
Another conceivable reason for differences in the clitellum is--as
I also think is the case with the genital papillae--to prevent
hybridisation. That the sense of touch is delicate in these animals
seems clear from the abundant development of epidermal sense-organs.
It may be that the feel of the clitellum during union enables two
individuals of a given species to come together and prevents those
of different species from mating. In any case there is no positive
evidence that hybridisation does occur in this group of animals.
The astounding variability and yet constancy in a given species
of the genital papillae is in favour of regarding these organs as
tactile recognition marks; and it will be noted that they are not by
any means characteristic of some of the older types of earthworms.
Furthermore they are particularly conspicuous in such genera as
_Pheretima_, _Megascolex_ etc., which possess a large number of
species. In these of course mutual recognition would otherwise be
more difficult.
[Illustration: Fig. 12. Ventral view of _Pheretima solomonis_ to
show papillae which are to be compared with those of fig. 13. (x
2.)]
[Illustration: Fig. 13. Ventral view of _Pheretima sedgwickii_.
(x 2.)]