They are soft bodied animals thata usually have an external
or internal shell. The typical mollusk body plan has four parts: foot, mantle,
shell and visceral mass.
Gastropods are shell-less or single-shell mollusks that move
by using a muscular foot located on the ventral side.
Bivalves have 2 shells that are held together by one or two
powerful muscles.
Cephalopods are typically soft- bodied mollusks in which
thehead is attached to a single foot, and this is divided into tentacles or
arms.
Like the annelids, mollusks have true coeloms surrounded by
mesoderm tissue. They also have complex organ systems that carry out processes
such as respiration and excretion.
The body plan of most mollusks has four parts: foot, mantle,
shell, and visceral mass. The muscular foot takes many forms, including flat
structures for crawling, spade-shaped structures for burrowing, and tentacles
for capturing prey. The mantle is a thin layer of tissue that covers most of
the mollusk’s body, much like a cloak. The shell is made by glands in the
mantle that secrete calcium carbonate. The shell has been reduced or lost in
slugs and some other mollusk groups. Just beneath the mantle is the visceral
mass, which consists of the internal organs
Reproduction in mollusca
Mollusks
reproduce in a variety of ways. Many snails and two-shelled mollusks reproduce
sexually by external fertilization. They release enormous number of eggs and
sperm into the open water. The eggs are fertilized in the water and then
develop into free-swimming larvae. In tentacle mollusks and certain snails,
fertilization takes place inside the body of the female. Some mollusks are
hermaphrodites, having both male and female reproductive organs. Individuals of
these species usually fertilize eggs from another individual.
Respiration in mollusca
Aquatic
mollusks such as snails, clams, and octopi typically breathe using gills inside
their mantle cavity. As water passes through the mantle cavity, oxygen and
carbon dioxide diffuse over the surface of the animal’s gills. Land snails and
slugs do not have gills. Instead, they respire using a mantle cavity that has a
large surface area lined with blood vessels. Because this lining must be kept
moist so that oxygen can diffuse across its surface, land snails and slugs
typically live only in moist places.
Phylum Arthropoda
Arthropods have a segmented body, a tough exoskeleton, and jointed appendages.
In many groups of arthropods, continuing evolution has led to fewer body segments and highly specialized appendages for feeding, movement and other functions.
When they outrgrow their exoskeletons, arthropods undergo periods of molting.
Arthropods are classified based on the number and structure of their body segments and appendages, particularly their mouthparts:
Crustaceans typically have two pairs of branched antennae, two or three body sections and chewing moutharts called mandibles.
Chelicerates have mouthparts called chelicerae and two body sections, and most have four pairs of walking legs.
Uniramians have jaws, one pair of antennae, and unbranched appendages.
Insects:
They have a body divided into three parts: head, torax and abdomen. Three pair of legs are attached to the torax.
The growth and development of insects usually involve metamorphosis, which is a process of changing shape and form.
Ants, bees, termites and other relatives form complex associations called societies.
The evolution of arthropods has led to fewer body segments and highly specialized appendages for feeding movement, and other functions.
A typical primitive arthropod was composed of many identical segments, each varying a pair of appendages. Its body probably closely resembled that of a trilobite. This early body plan was modified gradually. Body segments were lost or fused over time. Most living arthropods, such as spiders and insects, have only two or three body segments. Arthropod appendages also evolved into different forms that have different functions.
Reproduction in arthropods
Terrestrial arthropods have internal fertilization. In some species, males have a reproductive organ that places sperm inside females. In other species, the males deposit a sperm packet that is picked up by the females. Aquatic arthropods may have internal or external fertilization. External fertilization takes place outside the female’s body. It occurs when females release eggs into the external environment and males shed sperm around the eggs.
Respiration in arthropods
Most terrestrial arthropods breathe through a network of branching tracheal tubes that extend throughout the body. Air enters and leaves the tracheal tubes through spiracles, which are small openings located along the side of the body. Other terrestrial arthropods, such as spiders, respire using book lungs. Book lungs are organs that have layers of respiratory tissue stacked liked the pages of a book. Most aquatic arthropods, such as lobsters and crabs, respire though organs called book gills.
