Memoirs: Studies on Earthworms
By
0
land Benham, B>Sc, Demonstrator in the Zoological Laboratory of University College
,
London
-
Amongst the earliest papers dealing with Earthworms
anatomically are those of Savigny (1) in 1820, and of Duges (2) in
] 828, who describe numerous species of Lumbricus, which will
be mentioned in Section I I . Duges figures the prostomium of
some of these, and describes the genital organs; but his
interpretation of the latter is wrong, since he has, like so many
of the earlier writers, confused the seminal reservoirs and
the spermathecse, attributing each to the wrong sex. Other
authors followed him, who, whilst contradicting him, were no
nearer the truth. Von Siebold (3), for instance, suggested
that the ovary was invaginated into the seminal reservoirs.
Even till quite recently the " seminal reservoirs " were spoken
of as " testes." I may at once say that I shall use the former
name for the three pairs of large white organs in Lumbricus
which originate in somites x and xi and spread into the
neighbouring somites, and for their homologues in other genera.
The ovary was unknown till 1853, when d'Udekem (4)
described it in L u m b r i c u s a g r i c o l a j whilst in 1856 Hering
(5) supplemented our knowledge of the genital organs by his
figure of the ovary and his description of its position on the
posterior face of the septum between somites xn and XIII. He
also showed that the oviduct was not in continuity with the
ovary, but that the ova fell into the body cavity, and were
conveyed thence to the exterior by the wide ciliated funnels of
the pair of short oviducts which pass through the posterior
septum of somite XIII to the exterior in xrv. Hering described
the process of copulation, and thought that the spermatozoa
passed from the sperm pore along a groove of the ventral
surface to the spermathecse; but Dr. Fraisse, in 1882 (6), describes
the spermatophores of various spcies of Lumbricus, and shows
that the spermatozoa do not pass directly into the spermathecse,
but are received in bodies secreted on somite xxvi. Previous
authors had described as " testes" the large white sacs which
are now known as " seminal reservoirs," but Hering, in this
paper, describes and figures the true t e s t e s . Professor A. G.
Bourne was the first to figure them in their true position
attached to the anterior septum of somites x and xi, as two
pairs of small flat appendices. This figure and description occur
in a paper by J. E. Blomfield (7), who describes the
development of the spermatozoa in the reservoirs.
The sperm d u c t s were rightly described by Leo (8) in.
1820, but Duges (2) wrongly considered them as oviducts.
The n e p h r i d i a or "segmental organs" also were
erroneously interpreted by Dr. Williams in 1858 (40), being
considered as respiratory organs. Their true function was first
suggested by d'Udekem (9) in his description of Tubifex, whilst
Gegenbaur (10) in 1853 published the well-known drawing of
this organ of L u m b r i c u s agrieola, the histological
structure of which was described by Claparede (11).
In regard to the c l a s s i f i c a t i o n of Earthworms, that of
Claparede (12) in 1862 is usually followed. He divides the
order Oligochseta into two families, Limicolse and T e r r i
colffi, but the characters of the latter, as opposed to the
former, were derived from the genus L u m b r i c u s only, and
now, since the investigation of other genera, no longer hold
true. These are his characters:
a. The possession of two ventral blood-vessels.
b. The presence of nephridia in the same somites with the
sperm ducts and oviducts.
c. The position of the clitellum far behind the male pores.
d. The presence of a vascular network on the nephridia.
Now, Perrier's genus P o n t o d r i l u s (13) and Perichseta
have no subneural blood-vessel [Microchseta1 resembles
these two genera in this respect], and very possibly others will
also be found without this vessel.
The position of the clitellum is now known to vary;
sometimes it is in front, sometimes around as well as behind the
male pore.
The truly distinctive characters of the Terricolas (or
Lumbricinse, as Perrier calls them), as opposed to the
Linricolss, are the following:
1 Names or sentences in square brackets refer to results of my own research.
a. The presence of nephridia in the same somites with the
genital ducts (except in some species of Perichseta and
Pleurochseta (Megascolex), where nephridia are unknown
in any somite, and in P o n t o d r i l u s , in which the nephridia
are said not to commence1 till the hinder region of the sperm
duct, so that there are none in the somite carrying the oviduct).
b. The abundant vascular network on the nepridia and body
wall.
c. The almost universal presence of a gizzard
(Pontod r i l u s is again an exception).
d. The much smaller size of the ova and the compactness of
the ovary.
But even these charcters may have to be altered as new
forms are studied.
These Lumbricinas Perrier divides into (...truncated)