Coelomates. Arthropoda
The blood vascular system of arthropods is open. The
coelom is much reduced, and most of the spaces in the
arthropod body are hemocoels. The tubular heart is dorsal
and contained in a pericardial sinus. Blood is pumped
from the heart through a series of vessels (arteries)
that lead to the tissue sinuses. Although the blood
flows freely through the tissues it may, especially
in the larger species, be directed by membranes along
a more or less constant pathway. The blood collects
in a ventral sinus from which it is conducted back to
the heart through one or more venous channels.
Variations in the circulatory patterns of the different
classes of the phylum Arthropoda largely reflect the
method of respiratory exchange and consequent function
of the blood vascular system. Most of the aquatic species
of the class Crustacea have gills with a well-developed
circulatory system, including accessory hearts to increase
blood flow throughthe gills. A small number of species
lack gills and a heart, and oxygen is absorbed through
thebody surface; bodily movements or peristaltic gut
contractions circulate the blood within the tissue spaces.
In the mainly terrestrial class Insecta, the role of
oxygen transport has been removed from the blood and
taken over by the ramifying tracheal system that carries
gaseous atmospheric oxygen directly to the consuming
tissues. Insects are able to maintain the high metabolic
rates necessary for flight while retaining a relatively
inefficient circulatory system.
Among the chelicerate (possessing fanglike front appendages)
arthropods (for example, scorpions, spiders, ticks,
and mites), the horseshoe crab, Limulus , has a series
of book gills (gills arranged in membranous folds) on
either side of the body into which blood from the ventral
sinus passes for oxygenation prior to return to the
heart. The largely terrestrial arachnids may have book
lungs that occupy a similar position in the circulatory
pathway, a tracheal system comparable to that of insects,
or, in the case of smaller species, reduced tracheal
and vascular systems in which contractions of the body
muscles cause blood circulation through the sinus network.
The legs of spiders are unusual because they lack extensor
muscles and because blood is used as hydraulic fluid
to extend the legs in opposition to flexor muscles.
The blood pressure of a resting spider is equal to that
of a human being and may double during activity. The
high pressure is maintained by valves in the anterior
aorta and represents an exception to the general rule
that open circulatory systems only function at low pressure.
