Reproduction of the earthworm Lumbricus terrestris
Earthworms are hermaphroditic, meaning that each individual has a full set of fully functional male and
female reproductive organs. Reproduction is only by sexual means. (It is true that if a worm loses its head
or tail that it may regenerate the missing parts, but cutting a worm in half is highly likely to generate two
new worms, since regrowing the head-end is a precarious process that often fails and regenerating.
Usually when a worm is cut in two, the longer piece regenerates, especially if it has only the tail to regrow).
Female organs: there is one pair of small pear-shaped ovaries attached ventrally to the posterior surface
of the intersegmental septum between segments 12 and 13 (septum 12/13). The ovaries are situated
near to the mid-line of the worm and their narrower free ends float almost horizontally in the coelomic
fluid. They are covered in the peritoneum (the layer of cells that lines the coelom). Eggs are shed from
the ovaries into the fluid-filled coelom where they are collected by the open funnels of the oviducts in
segment 13. The eggs are stored in blind-ending sac-like side-branches (such structures are called
diverticula, singular diverticulum) of each oviduct, called ovisacs. The oviducts open to the outside in
segment 14 via the female gonopores. There are two pairs of spermathecae that receive the
co-copulant's sperm. These are small, white, flask-shaped receptacles in segments 9 and 10 (one pair
per segment) and they open in the intersegmental grooves.
Male organs: There are two pairs of palmate (flattened with finger-like projections) testes attached to the
posterior surface of intersegmental septua 9/10 and 10/11. They are enclosed in sacs called testis sacs
(coelomic sub-compartments) within the coelom in which their free ends float in segments 10 and 11.
Each testis sac is formed from peritoneum and has three large lateral lobes called seminal vesicles where
the sperm mature, after being shed by the testes, and are stored. Open sperm funnels enter the testes
sacs and open opposite to the four testes in segments 10 and 11. These funnels are ciliated and collect
mature sperm from the testis sacs. The funnels lead into long coiled tubes (called vasa efferentia). The
two vasa efferentia on each side of the worm (4 altogether) merge in segment 12 into straight tubes (of
which there is one pair) called vasa deferentia which open to the outside on segment 15 via the male
gonopores. From the male gonopores shallow seminal grooves (one pair) extend along the ventral
surface to the clitellum.
Lumbricus terrestris mates on the surface at night, emerging either completely from their burrows, or
keeping the posterior end inside the burrow if a partner is found near to. They press their ventral
surfaces together with segments 9, 10 and 11 of each worm opposite to the clitellum of the partner worm.
Each worm secretes a mucus tube around itself, covering segments 11 to 31. These tubes prevent
mixing of the sperm from one worm with that from another (which would reduce the efficiency of sperm
transfer). The tubes are especially tight around the clitella. Seminal fluid is released from the male
gonopores in segment 15 and squeezed along the seminal grooves to the clitellum of each worm and
from there into the spermathecae (segments 9, 10) of the opposite worm. (Arciform muscles present in
the body wall of segments 15 to 31 squeeze the sperm along the grooves).
During copulation the elongated and especially pointed copulatory chaetae on segments 10 to 15, 26
and 32 to 37 or on segments 10, 26, 31-38 and sometimes also segment 25 (descriptions differ from
different sources) are pressed into the co-copulant, piercing its body wall. Not only do these help to
maintain the grip of one worm upon the other, but recent research has shown that glands in these
chaetae inject fluid beneath the skin of the partner worm. Experiments have shown that the injected fluid
contains some sort of active substance which causes a change in the recipient worm which seems to
encourage the worm to distribute the sperm it receives more evenly between all four of its spermathecae
and may also increase the refractory period (the time interval between mating and mating again, most
probably with another worm). These changes seem to be a way in which the worm manipulates its partner
to reduce competition from other worms who may mate subsequently and use empty spermathecae in
which to deposit their sperm. This helps ensure that the partner remains 'faithful' and fertilises its eggs
with the current partner's sperm.
Segments 9, 10 and 11 also have ventral thickenings visible as whitish glandular areas on the underside
of the worm. These secrete sticky secretions that assist in grasping the partner during copulation. The
long copulatory chaetae of these segments are just outside these glandular prominences.
This mechanism of injecting the partner (and wounding it in the process) is best known in another
hermaphrodite - the land snail Helix aspera which deploys 'love darts' which it fires from the genital pore
(on the right side of its head) into its partner, shortly after the completion of courtship. Both partners
exchange darts. The darts carry mucus which contain an active chemical - a chemical signal, which
triggers the closing off the entrance to the partner's bursa copulatrix and the diversion of more sperm to
the spermathecae. (The bursa copulatrix is a pouch which takes up some of the recipients sperm and
digests them, so closing it off increases the success of sperm transfer). These snails are also
After copulation each worm produces cocoons for up to 12 months. In experiments in which subsequent
copulations were prevented, it was shown that the hatchability of these cocoons declines over this 12
month period with none observed to hatch after 7 months. Cocoon production was observed to peak at
2-3 months after mating. Each worm produced an average (median) of 5 viable cocoons after a single
mating. If allowed to do so, then earthworms will mate again, most likely with a different partner, whilst still
producing viable cocoons from a prior mating. This immediately raises the possibility of rivalry - a
subsequent partner may attempt to displace stored sperm from a prior mating from its partner.
When a cocoon is about to be produced, the worm secretes a mucus tube covering segments 8 to 38.
The clitellum then secretes the cocoon inside this mucus tube. About a dozen eggs are passed along
from the female gonopores (segment 14) to the cocoon (using the seminal grooves or adjacent outer
grooves?). The cocoon contains albuminous fluid to nourish the developing worm. The cocoon is pushed
forwards along the worm by expansion of the segments immediately behind it (peristalsis). When it
passes the spermothecal openings (segments 9/10 and 10/11) sperm are injected into it. Fertilisation of
the eggs then occurs inside the cocoon.
Eventually the cocoon is formed free over the front end of the worm and its elastic ends close. It darkens
and hardens to a lemon-shaped vessel about 5 mm long. Normally only one young worm emerges from
Above: an earthworm lying on its front has been dissected open from the dorsal surface and we
are looking down upon the internal structures of segments 9 to 15 - the segments containing the
reproductive apparatus. There are a pair of metanephridia in each segment (though only those
in segments 13, 14 and 15 are shown) - these are the earthworm equivalent of kidneys and
have an excretory and osmoregulatory function. Let us remove the gut (and associated blood
vessels) to see what lies underneath (we shall remove the metanephridia also):
The large testis sac, which has 3 pairs of lobes called seminal vesicles, contains the testes.
There are four testes in all, one pair in segment 10 and one pair in segment 11. Let us cut
windows into the testis sac and look inside: