Download PHYLUM ANNELIDA The Segmented Worms. There are

Annelids and Mollusks
Review
PHYLUM ANNELIDA The
Segmented Worms.
There are approximately 9,000 species
of segmented worms. The most familiar
are the earthworms and the freshwater
leeches. Two thirds of the phylum are
much less familiar marine, polychaete
worms. The principal characteristic
feature of these animals is their
external separation into repeating units
or segments. Each segment is also
termed a metamere.
PHYLUM ANNELIDA The
Segmented Worms.
Annelids have a worldwide distribution. They
are found in marine, freshwater and
terrestrial habitats. In terrestrial
environments, the worms cannot be removed
from moisture for an extended period of time
or they will dry up. Annelids are either
herbivores or carnivores. The earthworms,
for example, feed upon organic matter in the
mud, while many of the marine forms are
active hunters. The leeches are ectoparasites
of vertebrate animals.
Phylum Characteristics:
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1. Multicellular, bilaterally symmetrical, segmented worms.
2. They are at the organ-system level of organisation.
3. The annelids are triploblastic, schizocoelomate protostomial animals.
4. With the exception of the leeches, each of the annelid worms has chitinous setae
(chaetae) on their latero- ventral surface.
5. The coelom is separated at each segment by septa.
6. The digestive tract is complete.
7. The body is covered with a moist cuticle secreted by the epidermis.
8. Osmoregulation and excretion are done by paired nephridia (one pair. Per
segment).
9. Respiratory gas exchange is through the skin, gills or fleshy appendages,
parapodia.
10. Blood is circulated through a closed circulatory system and the oxygen is
transported on respiratory pigments.
11. The nervous system consists of a double ventral nerve cord, a pair of ganglia and
lateral nerves per segment. The `brain' consists of a pair of dorsal cerebral ganglia,
circumpharyngeal connectives and a pair of ventral subpharyngeal ganglia.
12. The muscles of the body wall and intestinal tract pull against the hydrostatic
skeleton of the coelom.
13. The segmented worms are hermaphroditic or with separate sexes. Fertilization is
external and many produce trochophore larvae.
PHYLUM MOLLUSCA:
This is the largest phylum (or division) of living
organisms outside of the Arthropoda. It includes
such organisms as the snails, oysters, clams,
squid and octopuses. There are approximately
50,000 extant species and 35,000 fossil species
now classified. They range in size from
microscopic to the giant squids with a body
length of 18m and a long tentacle length of an
additional 50m. The mollusks are primarily
aquatic organisms found in almost all marine
and freshwater habitats. They also live in
terrestrial environments although they avoid
direct sunlit areas. A wide variety of mollusks are
used as food. Others are more harmful,
damaging wooden ships and wharves
(shipworms) and eating vegetable crops (slugs).
Phylum Characteristics:
• 1. These multicellular animals are bilaterally symmetrical or develop
into a form of asymmetry.
• 2. They have reached an organ system level of organization.
• 3. The molluscs are triploblastic, protostomial schizocoelomates.
• 4. The ventral body wall is modified into a foot and the dorsal body
wall produces two tissue folds, the mantle.
• 5. They have a complete digestive system with extracellular
digestion.
• 6. The outside of most of these animals is covered with a protective
shell. The epidermis is usually ciliated with many mucous glands.
• 7. The coelom is reduced to a small cavity around the heart.
• 8. The open circulatory system has a heart, several major arteries,
blood sinuses and respiratory blood pigments.
• 9. Gas exchange is across gills, lungs or the mantle wall.
• 10. Excretion is through the use of metanephridia.
• 11. The nervous system of most consists of 3 or 4 pair of ganglia,
interconnecting fibers and sensory cells.
• 12. The muscles pull against the shell or a hydrostatic skeleton.
• 13. The animals are dioecious and reproduce only sexually.
Class Bivalva
Class Cephalopoda