7th Grade Science Student ebook

UNIT 5 STRUCTURE AND FUNCTION IN LIVING THINGS

Figure 15.14: Representative

mollusks.

Mollusks

The Phylum

Mollusca

Snails, clams, and squids are all members of the Phylum

Mollusca—the mollusks (Figure 15.14). Mollusks are more

advanced than roundworms but not as advanced as annelids.

There is great diversity among mollusks, but each type of mollusk

has a similar body plan. Mollusks have bilateral symmetry

and a true body cavity. The body of a mollusk typically has

a foot, gut, mantle, and shell.

The mollusk body The foot is a soft, muscular
structure that usually contains
the mouth. The mouth may have
a feeding structure called a
radula. The gut is the mollusk’s
digestive tract. The mantle is a
thin layer of tissue that
surrounds part of the mollusk’s
body. Glands in the mantle
secrete calcium carbonate, a
compound that makes up the
shell. Some mollusks do not have
a shell but may have evolved from ancestors that had a shell.

Other organ

systems and

reproduction

Mollusks have a circulatory system with a simple heart. They also

have a nervous system. Clams and their relatives have a simple

nervous system with nerve cords and a few ganglia. Octopi and

their relatives have a more advanced nervous system. They have a

well-developed brain and eyes. They even have a sense of touch

and taste. All mollusks reproduce sexually and have a life cycle

that includes a stage called a larva. Some mollusks are

hermaphrodites.