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Phylum Mollusca
Mollusks have several characteristics:
A trocophore larva
A coelom - a true coloem develops completely within the mesoderm, which allows
contact between the mesoderm and endoderm and therefore, complex organ
systems. In mollusks, it's often just a space around the heart.
Bilateral symmetry
Three body parts:
o a mantle, which is a heavy fold of tissue as the outer body layer;
o a visceral mass, which is the central area containing organs; and
o a muscular foot, which is mainly used for locomotion.
A radula, which is a rasping, tongue-like structure in the mouth used for scraping
or attacking prey.
Mollusks have organ systems for:
o digestion,
o reproduction - some gastropods are hermaphrodites, but most others have
distinct males and females. Some oysters and sea slugs can change sex.
Cephalopods and freshwater snails do not have free-swimming larvae.
o circulation - cephalopods have a closed circulatory systems, but others have
a 3-chambered heart and an open system.
o respiration - most have gills, but terrestrial gastropods have a primitive lung
that requires a certain humidity or moisture
o excretion - including nephridia (found in all coelomates except arthropods
and chordates
A shell of protein strengthened by calcium carbonate (mollusks w/out shells
include squid, cuttlefish, octopus, and slugs).
Gastropods have a single shell or none at all (slugs and nudibranchs).
Terrestrial specias secrete a slippery path to move along.
Many are pests, and some are predators (whelks and cons shells). Terrestrials are
not usually eaten, but some marine species are prized.
Bivalves are mostly marine, but North America has many freshwater mussels.
Bivalves have no radula or a distinct head.
A nerve ganglion above the foot functions as a primitive brain, and there are
simple sense organs
Many mollusk larvae are parasitic.
Most adults are filter feeders and have siphons, using their gills to trap foo)
Bivalves coat foreign substances coat foreign materials with nacre (mother of
Cephalopoda (Squids, Octopuses, Cuttlefish and Nautiluses)
They have a large head, and the foot is divided into tentacles.
Squid have 10 tentacles, octopuses have 8, and the nautilus has 80-90.
The nautilus is the only cephalopod with an outer shell. Squid and cuttlefish have
an internal "shell"
They are considered the most intelligent invertebrates.
Like other aquatic mollusks, the draw water into their mantle cavity and expel it
through their siphon.
Cuttlefish ink is reddish brown pigment called sepia and was used by artists.
All are predators. The tentacles have suction or hooks, and they have strong beaks
and a radula.
Phylum Annelida
Some annelids also have trocophore larvae.
Annelid fossils have been found in rocks over 530 million years old.
Some are terrestrial earthworms, but most are marine.
They range from 1mm to 3 m (10ft.)
Annelids have a coelom, a closed circulatory system, nephridea, and a highly
modified digestive gut.
Annelids are the first segmented animals. Each segment has organs for digestion,
excretion, and circulation. Some segments are specialized for reproduction,
feeding, or sensory functions.
One anterior segment contains a cerebral ganglion, connected to a nerve chord
running along the underside of the worm.
Internal body division or walls are called septa, with a circulatory system that
passes through.
Most have external bristles called setae, and some have parapodia. This is how
annelids are classified.
Class Polychaeta have representatives in almost all marine habitats. They all have
fleshy appendages called parapodia, which are used for locomotion and gas
Some polychaetes ("many hairs")live in tubes they created with secretions, others
burrow in the sand, and some are free-swimming.
They have a distinct head, and some like Nereis have pichers for capturing prey.
Class Oligochaeta (earthworms and some freshwater worms)
They have no parapodia, no eyes, and no distinctive head region. But they do have
organs that sense light and moisture, a specialized digestive tract (with a gizzard),
and a hydrostatic skeleton.
As long as the skin is moist, oxygen can be diffused directly through the skin.
The earthworm, Lumricus terrestris, has a clitellum, which secretes a mucus
caccoon for fertilized eggs.
Class Hirudinea (leeches)
No seta or parapodia.
Body segments are not sepearated internally by septa, and the body is flattened.
They are increasingly used after surgery, particularly microsurgery for severed