IB-202-4. 3-15-06. Most animals have bilateral symmetry The vast majority of animal species belong to the clade Bilateria Which consists of animals with bilateral symmetry and triploblastic development. Developmental Patterns (Deuterostome and Protostome).
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(examples: molluscs, annelids,
(a) Cleavage. In general, protostomedevelopment begins with spiral, determinate cleavage.Deuterostome development is characterized by radial, indeterminate cleavage. Means at the 8 cell stage you
can separate out a cell and will develop into a complete individual
Spiral and determinate
Radial and indeterminateCleavage
(b) Coelom formation. Coelom formation begins in the gastrula stage. In protostome development, the coelom forms from splits in the mesoderm (schizocoelous development). In deuterostome development, the coelom forms from mesodermal outpocketings of the archenteron (enterocoelous development).
folds of archenteron
masses of mesoderm
split and form coelom
Figure 32.9bCoelom Formation
a circulatory system as diffusion is adequate to meet nutrient distribution from the gut, gas exchange and waste disposal needs because of short distance from cell to environment
the cells lining the gastro-
vascular cavity, which has
three branches, each with
fine subbranches that pro-
vide an extensive surface area.
Pharynx. The mouth is at the
tip of a muscular pharynx that
extends from the animal’s
ventral side. Digestive juices
are spilled onto prey, and the
pharynx sucks small pieces of
food into the gastrovascular
cavity, where digestion continues.
through the mouth.
Ganglia. Located at the anterior end
of the worm, near the main sources
of sensory input, is a pair of ganglia,
dense clusters of nerve cells.
Ventral nerve cords. From
the ganglia, a pair of
ventral nerve cords runs
the length of the body.
intestine. A female fluke fits into a groove running
the length of the larger male’s body, as shown in
the light micrograph at right.
These larvae penetrate
the skin and blood
vessels of humans
working in irrigated
with infected human
Blood flukes reproduce
sexually in the human host.
The fertilized eggs break through
blood vessel wall into the digestive
tract and exit the
host in feces.
The eggs develop in
water into ciliated
larvae. These larvae
infect snails, the
within a snail results in
another type of motile
larva, which escapes from
the snail host.
Figure 33.11Life cycle of blood fluke (Schistosoma)
of the earthworm is
partitioned by septa. Allow
of body wall.
segment of the worm
contains a pair of
excretory tubes, called
ciliated funnels, called
wastes from the blood
and coelomic fluid
through exterior pores.
Each segment is surrounded by longitudinal muscle, which in
turn is surrounded by circular muscle. Earthworms coordinate
the contraction of these two sets of muscles to move (see
Figure 49.25). These muscles work against the noncompressible
coelomic fluid, which acts as a hydrostatic skeleton.
Many of the internal
structures are repeated
within each segment of
Chaetae. Each segment
has four pairs of
chaetae, bristles that
provide traction for
Tiny blood vessels are
abundant in the earthworm’s
skin, which functions as its
respiratory organ. The blood
Cerebral ganglia. The
earthworm nervous system
features a brain-like pair of
cerebral ganglia above and
in front of the pharynx. A ring
of nerves around the pharynx
connects to a subpharyngeal
ganglion, from which a fused
pair of nerve cords runs
Giant Australian earthworm
Ventral nerve cords with segmental ganglia.
The nerve cords penetrate the septa and run
the length of the animal, as do the digestive
tract and longitudinal blood vessels.
The circulatory system, a network of vessels,
is closed. The dorsal and ventral vessels are linked
by segmental pairs of vessels. The dorsal vessel
and five pairs of vessels that circle the esophagus
of an earthworm are muscular and pump blood
through the circulatory system.
Can acquire this parasite from eating uncooked pork of infected swine. The encysted juveniles mature and migrate from your intestine into the intestinal muscle. Produce more juveniles which migrate to the heart where they encyst forming calcium deposits. Avoid the immune system. Make muscle cell bigger to house them and vascularization. Painful inflammation.
system. The dorsally located heart pumps
circulatory fluid called hemolymph through arteries
into sinuses (body spaces). The organs of the
mollusc are thus continually bathed in hemolymph.
Nephridium. Excretory organs
called nephridia remove metabolic
wastes from the hemolymph.
The long digestive tract is
coiled in the visceral mass.
Radula. The mouth
region in many
contains a rasp-like
called a radula. This
belt of backward-
curved teeth slides
back and forth,
scooping like a
of a nerve ring
Figure 33.16Generalized Anatomy of a mollusc
Figure 33.19Torsion in Gastropods
have flattened shells (abalone). Head with eye stalks. Modified radula as poisonous dart (marine cone snail toxin can kill humans).
most intelligent invertebrates.
(b) Squids are speedy carnivores with beaklike jaws and well-developed eyes.
(c) Chambered nautiluses are the only living cephalopods with an external shell. Have rudimentary eye without lens like a pin hole camera. Shell is chambered and put less dense ammonium chloride in chamber for flotation. Less dense than seawater.