Download Phylum Mollusca and annellida

Phylum Mollusca
Molluscs are bilaterally symmetrical eumetazoans. They have a true coelom (eucoelom)
which is formed by the splitting of embryonic mesodermal masses (schizocoely).
Development is protostomous.
An important characteristic of most molluscs is the head-foot region. Most molluscs are
strongly cephalized; that is, they have a well-developed head, in which is located a mouth
and a concentration of nervous and sensory functions. Adjacent to the head is a large,
muscular foot formed from the ventral body wall. Used primarily in locomotion, the
surface of the foot is sometimes ciliated and laden with numerous mucous glands.
Another characteristic of most molluscs is the mantle. This sheath of tissue is formed
from the dorsal body wall. It surrounds the mantle cavity, which houses the gills or lungs
if they are present, and its surface may assist in gas exchange. The mantle also secretes
the shell in those forms that possess one.
Internally, molluscs have a complicated digestive system, with a mouth in the head and
the anus emptying into the mantle cavity. A rasping organ used in feeding, the radula, is
present in all groups except bivalves and Aplacophora. The radula is usually toothed and
its structure may be very complex. It is adapted to a wide variety of feeding styles in
different species, including scraping, tearing, stabbing, and cutting.
Molluscs have an open circulatory system (except cephalopods, in which it is closed),
complete with a heart, blood vessels, and respiratory pigments. Gas exchange is via gills,
lungs, or the body surface. Excretion takes place by means of kidneys that, like the
digestive tract, pump waste into the mantle cavity.
A fairly complicated nervous system is present, including several ganglia and a system of
nerves; sensory and nervous systems are cephalized in at least some kinds of molluscs.
Some molluscs have complex, extremely sensitive eyes.
The majority of molluscs have a shell of some kind. This calcareous structure is secreted
by the mantle, and in living snails it is covered by a horny layer called a periostracum. Its
shape, size, and color are widely used by taxonomists, and they has also made molluscs a
popular pursuit of collectors. Further, shells decay slowly and fossilize well, and the
fossil record of molluscs is excellent and ancient.
Most molluscs are dioecious. Many pass through free-swimming larval stages, called
trochophore and veliger larvae.
Molluscs are an extremely diverse group of organisms. Over 50,000 living species have
been named, making Mollusca second only to the Phylum Arthropoda in number of
species known. The majority of molluscs are marine, but large numbers of species
occupy freshwater and terrestrial habitats. Molluscs are also extremely diverse in their
food habits, ranging from species that graze on microscopic algae, to those that eat the
leaves of terrestrial plants, to predators that capture fish and other molluscs. Many
species of molluscs are important to humans. A large number of bivalves and some snails
are important sources of protein. Oysters produce pearls. Other species are pests in
gardens and to crops. Some are essential components in the life cycles of human
parasites, including devastating diseases such as schistosomiasis. And a few, such as
oyster drills, cause problems by preying on other molluscs that are important to us.
Finally, molluscs play a wide variety of essential ecological roles. As common
herbivores, they can have a significant impact on the plant species present in an area. As
predators, they may have a similar effect on animals, especially other molluscs. As prey,
they provide food for a large number of organisms, including many vertebrates (some of
which, such as sea otters, are strikingly specialized for feeding on them). An example of
the ecological (not to mention economic) impact of molluscs is given by the recent
introduction of zebra mussels to the Great Lakes. Zebra mussels live in large numbers on
hard surfaces, and they feed by filtering particles from the water. The effectiveness of
their feeding is remarkable; water clarity in some of the Great Lakes has improved
dramatically in the years since zebra mussels first appeared. Unfortunately, the same
material that the mussels are efficiently removing is the basis of a complicated food chain
for other species, including economically-important game fishes. Fisheries biologists in
the Great Lakes states are very concerned about the future of the species they tend. The
impact of these changes on less conspicuous species, such as the many kinds of native
bivalves, is not known, but their populations are certainly at risk.
http://www.kent.wednet.edu/staff/rlynch/sci_class/chap14/mollusc.html
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Annelida
Phylum Annelida
The Segmented Worms
"Although a spacious, fluid-filled coelom provided an efficient hydrostatic skeleton for
burrowing, precise control of body movements was not possible in the earliest
coelomates. The force of muscle contraction was carried through the body by the fluid in
the undivided coelom. This defect was remedied when a series of partitions (septa)
evolved in the ancestral annelids. When the septa divided the coelom into a series of
compartments, components of most other body systems, such as circulatory, nervous, and
excretory, were repeated in each segment. This body plan is known as metamerism."
Hickman, C.P. and L.S. Roberts. 1994. Biology of Animals, Sixth Edition. Wm.C.Brown Publishers:
Dubuque, Iowa. Page 526.
Major Characteristics of Phylum Annelida
1. Body metamerically segmented; symmetry bilateral;
2. Body wall with outer circular and inner longitudinal muscle layers; outer
transparent moist cuticle secreted by epithelium;
3. Chitinous setae, often present on fleshy appendages called parapodia; setae
absent in leeches;
4. Coelom (schizocoel) well developed and divided by septa, except in leeches;
coelomic fluid supplies turgidity and functions as hydrostatic skeleton;
5. Blood system closed and segmentally arranged; respiratory pigments
(hemoglobin, hemerythrin, or chlorocruorin) often present;
6. Digestive system complete and not metamerically arranged;
7. Respiratory gas exchange through skin, gills, or parapodia;
8. Excretory system typically a pair of nephridia for each metamere;
9. Nervous system with a double ventral nerve cord and a pair of ganglia with lateral
nerves in each metamere; brain, a pair of dorsal cerebral ganglia with connectives
to cord;
10. Sensory system of tactile organs, taste buds, photoreceptor cells, and eyes with
lenses (in some);
11. Hermaphroditic or separate sexes; asexual reproduction by budding in some;
Classes of Annelids:
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Class Polychaeta - The polychaetes are the largest class of annelids, with more
than 10,000 described species, most of them marine. They differ from other
annelids in having a well-differentiated head with specialized sense organs;
paired, paddlelike appendages (parapodia) on most segments, and no clitellum.
Example: Nereis (the clamworm)
Class Oligochaeta - The more than 3,000 species of oligochaetes are found in a
great variety of sizes and habitats. Most are terrestrial or freshwater forms, but
some are parasitic and a few live in brackish or marine water. With few
exceptions, they bear setae, which may be long or short, straight or curved, blunt
or needlelike, or arranged singly or in bundles.
Examples: Lumbricus (earthworm) {go to Earthworm Dissection Home
Page}
for information about earthworms on another
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websites!

Class Hirudinea - Leeches, numbering over 500 species, are found
predominantly in freshwater habitats, but a few are marine, and some have even
adapted to terrestrial life in warm, moist areas. They are usually flattened
dorsoventrally, and are found in a variety of colors - black, brown, red, or olive
green. Leeches have a fixed number of segments, usually 34, and typically have
both an anterior and a posterior sucker.
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