Earthworm - Circulatory System
Above: a model showing the anterior end of the earthworm Lumbricus terrestris, with part of the wall cut
away to show the main blood vessels and some of the other principle organs. The gut is shown in yellow,
the nerve cord with its ganglia in green and the blood vessels in red. The intersegmental septa are
shown in grey, let us remove these septa for clarity:
Below is a labelled version of the above diagram. Note the dorsal vessel, a single structure, running
along the back (top) of the worm, above the gut and the single ventral vessel below the gut and the
single subneural vessel below the nerve cord. Smaller blood vessels (not shown) supply the body wall
and the gut. Note the pseudohearts (hearts) in segments 7 to 11 (there are five pairs in total, one pair
per segment) that connect the dorsal to the ventral vessel and the parietal vessels which connect the
dorsal vesel to the subneural vessel in each segment posterior to segment 11 (there was one pair of
these parietal vessels per segment).
The route of the main circulation is as follows:

Blood flows anterior (forwards) along the
dorsal vessel (dv), then downwards in the pseudohearts to the ventral vessel
(vv), both anterior and posterior, away from the pseudohearts, in the ventral vessel.

The detailed circulation is shown in the diagram below, which is a cross-section through the worm through a typical
segment posterior to the pseudohearts, e.g. segment 12, and is looking along the axis of the worm and superimposes
all the vessels in that segment onto the plane of the page. The central U-shaped structure is the intestine, as seen in
cross-section. The arrows indicate direction of blood flow.

The ventral vessel gives off smaller branches to supply the body wall (bw) and internal organs (nephridia (nph) which
are 'little kidneys' and gut/intestinal wall (ivp)). Blood vessels return blood from the body wall and nephridia  to the
parietal vessels (p, also called commissural vessels) and hence to the dorsal vessel. Blood vessels also convey blood
back from the body wall to the lateral neural vessels (ln), a pair of medium-sized blood vessels that run either side of the
backwards (posteriorly) in the subneural vessel. Blood is conveyed by vessels from the ventral vessel to a network of
vessels supplying the intestine wall (ivp). The typhlosole sinus is a blood filled space (a blood sinus is like a blood
vessel but without a definite cellular lining) that collects blood from the intestine wall (ivp) carries blood along the
typhlosole, which is a pendulous projection or ridge along the roof of the intestine. Blood flows from the typhlosole sinus
back to the dorsal heart through the dorso-typhlosolar vessel (dt). Additionally paired dorso-intestinal vessels  carry
blood from the intestine wall (ivp) back to the dorsal heart.
Essentially this pattern repeats itself in each of the 150 or so segments of the worm, except right at the front and
back ends where it is modified. The various ascending and descending vessels, such as the parietals, repeat in
each segment. However, the longitudinal vessels, such as the dorsal, ventral and subneural run along the whole
length of the worm, so the circulatory system is not entirely divided into segments. Indeed, despite the segmental
nature of some of the vessels, the circulatory system functions as a single whole, as blood is carried forwards
along the dorsal vessel and backwards along the ventral and subneural vessels - there is a proper single systemic
circulation.

This system has a series of pumps. The five pairs of
pseudohearts (sometimes called hearts) function like hearts
as they contract rhythmically, pumping blood down from the dorsal vessel to the ventral vessel. Valves close
should blood attempt to flow the wrong way (as when the pressure drops when the vessel relaxes).

Some authours call the pseudohearts 'hearts', however, the large dorsal vessel is also contractile, pumping blood
forwards and this corresponds to the 'heart' of insects. Thus, if any part be called the heart, then perhaps it should
be the dorsal vessel. On the other hand, in vertebrates, such as humans, the dorsal vessel (the dorsal aorta) is
contractile but the anterior heart is clearly the main pump, so by comparison with vertebrates the pseduohearts
become the hearts. Perhaps this illustrates the problems with using the names for vertebrate organs for those of
invertebrates. Personally, I prefer the term pseudoheart as this implies only a loose resemblence to the vertebrate
heart.

The structure of part of the dorsal vessel (in longitudinal section) as spanning one segment plus a bit is shown
below. Note that this too has valves to prevent the back-flow of blood. The lower part is thinner as this part is
contracting (notice how the valves to the side vessels that supply the dorsal vessel are shut tight in this region, to
prevent blood flowing out of the dorsal heart along the dorso-intestinal vessels.
The diagram below illustrates a cross-section through a pair of pseudohearts, notice the valves and the direction
of blood flow (as indicated by the arrow in the left pseudoheart).
Blood flows into the head of the worm along the dorsal vessel in front of the pseudohearts and then passes back
along a pair of vessels, one on either side of the foregut (oesophagus) called lateral oesophageal vessels and in
the subneural vessel which carry the blood back behind the hearts and back to the dorsal vessel along the
parietals of segment 12 and later segments.

Note that, as in humans, the circulatory system of earthworms is a
closed system - blood can complete a whole
circuit whilst remaining confined within blood vessels. This contrasts with the
open circulation of insects, in which
blood leaves the main vessels and baths the fluid in large blood-filled spaces or sinuses (the haemocoel). In reality
each system mixes these two types - insects do have blood vessels to convey blood to the antennae, but the blood
still leaves the ends of these vessels. In mammals, some organs have an open blood circulation, such as the
sinuses of the liver where blood directly bathes the tissue. Earthworms, like vertebrates, possess tiny blood vessels
called capillaries which connect the larger vessels carrying blood towards and away from the tissue. In the body
wall, these capillaries form loops which absorb oxygen which diffuses in across the skin from the outside air. The
blood of earthworms carries oxygen, nutrients and waste products around the worm's body.

The blood contains only white blood cells - amoeboid cells (phagocytes) that engulf and destroy cell debris, foreign
organisms and waste products, and so help protect and maintain the worm. There are no red blood cells
(erythrocytes), as in vertebrates, instead the red oxygen-carrying pigment
haemoglobin is carried in solution as
erythrocruorins - giant extracellular respiratory complexes of 144 haemoglobin protein chains arranged around a
central scaffold of 36 protein subunits. They are arranged around the scaffold in 12 groups of 12 chains, and in
each group the chains are arranged in three groups of four. The chains are held together by linker proteins.

Earthworms frequently encounter low oxygen levels in soil and there haemoglobin is adapted to scavenge oxygen
for which it has a much higher affinity than human haemoglobin. In
Lumbricus, the haemoglobin is 50% saturated
with oxygen at an oxygen pressure below 10 mm Hg compared to above about 20 mm Hg for a human. However, if
soils become water-logged then the earthworm is in danger of drowning and will abandon its burrow and come to
the surface.

References:

The diagrams of the dorsal vessel and the plan of segmental circulation were redrawn (and simplified) from:

J. B. JOHNSTON. On the blood vessels, their valves and the course of the blood in Lumbricus, Biol. Bull., 1903, 5: 74-84. (Available
online as a PDF).