As protostome coelomates, the crustacean body cavity is a
coelom and, during embryological development of protostomes,
the mouth originates at or near the blastopore, the opening
from the outside of the blastula to its interior. The crustacean
body plan resembles that of other arthropods in some respects
(for example, nervous system), but most crustaceans have three
body sections—head, thorax, and abdomen—as do insects; in
comparison, chelicerates have two body sections (fused cephalothorax
and abdomen). In some crustaceans, the head is fused
with various thoracic segments to form a cephalothorax. The
crustacean coelom is small and the main body cavity—a hemocoel,
or cavity in which blood circulates—is part of the circulatory
system, as in other arthropods. The crustacean mode of
gas exchange, however, differs from that in other arthropods;
gills—evident in crabs and lobsters—exchange gases with the
water or air. A tube-shaped heart with ostia is the circulating
pump for blood. In lobsters, one of the mouthparts is modified
as a gill bailer, which the animal uses to generate a current of
water through its gill chamber, enhancing gas exchange through
the gills. Crustaceans that are semiterrestrial (for example, the
hermit crab Clibanarius) inhabit the intertidal zone of the sea
beach, a habitat that provides at least occasional moisture to
their respiratory surfaces. Some crustaceans have blood vessels;
others have no vessels and pump blood only to the hemocoel.
As protostome coelomates, the crustacean body cavity is a
coelom and, during embryological development of protostomes,
the mouth originates at or near the blastopore, the opening
from the outside of the blastula to its interior. The crustacean
body plan resembles that of other arthropods in some respects
(for example, nervous system), but most crustaceans have three
body sections—head, thorax, and abdomen—as do insects; in
comparison, chelicerates have two body sections (fused cephalothorax
and abdomen). In some crustaceans, the head is fused
with various thoracic segments to form a cephalothorax. The
crustacean coelom is small and the main body cavity—a hemocoel,
or cavity in which blood circulates—is part of the circulatory
system, as in other arthropods. The crustacean mode of
gas exchange, however, differs from that in other arthropods;
gills—evident in crabs and lobsters—exchange gases with the
water or air. A tube-shaped heart with ostia is the circulating
pump for blood. In lobsters, one of the mouthparts is modified
as a gill bailer, which the animal uses to generate a current of
water through its gill chamber, enhancing gas exchange through
the gills. Crustaceans that are semiterrestrial (for example, the
hermit crab Clibanarius) inhabit the intertidal zone of the sea
beach, a habitat that provides at least occasional moisture to
their respiratory surfaces. Some crustaceans have blood vessels;
others have no vessels and pump blood only to the hemocoel.
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