Mollusks are triploblastics, in which they have three germinal layers including the ectoderm (covering the outer body), the endoderm (lining the digestive tube), and a third layer, the mesoderm (which is a middle layer between the two).

Germ Layers

Mollusks have bilateral symmetry (symmetry in which you can divide an organism into equal right and lefts sides).

Symmetry

Definition: fluid filled cavity that develops within the mesoderm.

It functions as a hydrostatic skeleton and provides space within the internal organs. Mollusks are said to have a “true coelom”.

Coelom

The digestive tract is complete with a mouth, anus and complex stomach. The pattern of the stomach varies according to the mollusks diet.

Digestive Tract

Food is taken up by cells lining the digestive glands from the stomach, and then is passed into the blood. Undigested materials are compressed and packaged, then discharged through the anus into the mantle cavity and are carried away from the animals in the water currents. This packaging of wastes in solid form prevents fouling of the water passing over the gills.

Digestive Tract cont.

Definition: a tendency in the development of animals to localize important organs or parts in or near the head. It is the process in which nervous and sensory tissues of Mollusks become concentrated in the head.

In the different classes of Mollusks, the cephalization is usually well developed.

Cephalization

Definition: The division of an organism into several repeated segments.

Mollusks do not exhibit segmentation in their bodies.

Segmentation

Mollusks have a soft body and most of them are protected by a shell. Some of the mollusks lost their protective shell through evolution.

They have a muscular "foot" that is used for movement.

In some mollusks, such as an octopus, as many as four pairs of appendages are present.

Skeleton & Appendages

Mollusks have an open circulatory system, meaning that the blood does not circulate entirely within vessels.

It is collected from the gills, pumped through the heart, and released directly into the tissues. It is then returned to the gills, and then back to the heart.

Circulatory System

Aquatic mollusks (i.e. snails, clams and squid) breathe through the gills inside their mantel cavities, but land mollusks use an adapted mantle cavity lined with blood vessels to breath ("skin breathing“).

Respiratory System

Mollusks reproduce sexually either through external fertilization (egg is fertilized outside of the body) or through internal fertilization (egg is fertilized inside the body.)

Reproduction

All mollusks produce eggs , which hatch into larvae.

Some mollusks are hermaphrodites, in which they function as both sexes.

Mollusks can be found in the oceans and on land. The only environment that they cannot live in is very dry environments because their skin easily loses its moisture.

Habitat

There are about 160,000 species of mollusks, in which 128,00 are living and 35,000 are fossil records.

They were discovered during the Cambrian Explosion.

The giant squid and the giant clam are among the world's largest mollusks.

Fun Facts!