This image (and the three close-up views linked above) is taken from a plastic model of
an earthworm. Earthworms burrow through soil, feeding on decaying organic
matter. As pointed out by Charles Darwin, they perform a beneficial role by
aerating the soil and enriching it by bringing up nutrients from below.
In terms of
nutrition, food is brought into mouth by a muscular pharynx that passes through
the esophagus, which is surrounded by calciferous glands that remove excess
calcium ions (acquired from eating the soil) from the blood and secrete them
into the gut. Food then enters a thin-walled crop and is passed on to a
muscular gizzard, which serves as a grinding organ. From there it passes into
the intestine to exit out the terminal anus. The dorsal wall of the
intestine is folded inward to form a typhlosole that serves to increase the
surface area for digestion and absorption. Chloragogue cells surrounding the
intestine and filling much of the typhlosole are involved in the synthesis of
glycogen and fats.
Although much of the circulation in annelids is handled by the coelom,
earthworms also have a well-developed, closed circulatory system consisting of a
dorsal vessel that runs above the alimentary canal from the anus to the pharynx.
The dorsal vessel receives blood from the body wall and pumps it anteriorly into
five pairs of aortic arches that help maintain a steady pressure into the
ventral vessel, which delivers blood to the rest of the body.
In terms of excretion, some wastes simply diffuse out through the moist skin,
which also serves as the principal gas exchange organ. Other wastes are handled
by paired structures called nephridia. Each nephridium (also called a
metanephridium) has a ciliated funnel-like nephrostome that collects wastes from
the coelomic fluid and then passes it through the transverse septum into the
next metamere. The nephridia empty to the outside via a openings called
In terms of
although earthworms are monoecious, they practice cross fertilization.
Copulation occurs between partners that are joined by mucus secretions from a
saddle-like structure called the clitellum and by special ventral setae that
penetrate each partner’s body. Sperm are released from the seminal vesicles of
one partner and received by seminal receptacles of the other after passing along
a seminal groove. After copulation, the clitellum of each worm secretes a cocoon
that receives the sperm and eggs, which are then fertilized in the cocoon. The
cocoon is then deposited in the ground, where direct development takes place,
terminating when a young earthworm that resembles the adult hatches from the