What are Mollusks?
Mollusks belong to the phylumMollusca. The mollusk body is often divided into a head
with eyes or tentacles, a muscular foot, and a mass housing the organs. In most species, the
muscular foot is used for locomotion. Mollusks also have a mantle, a fold of the outer skin
lining the shell, which in most mollusks secretes a calcium carbonate external shell, just like
the ones you find on the beach.
The majority of marine mollusks have a gill or gills to absorb oxygen from the water. All
species have a complete digestive tract that begins at the mouth and runs to the anus. Many
have a feeding structure, theradula, found only in mollusks. The radula is composed mostly
ofchitin, a tough, semitransparent substance that is the main component of the shells of
crustaceans and the outer coverings of insects. Radulae range from structures used to scrape
algae off rocks to the beaks of squid and octopuses.
Larval development suggests a close relationship between the mollusks and other groups, no-
tably the annelids, any of various worms or worm-like animals, including the earthworm and
leech, characterized by a cylindrical, elongated, and segmented body. Unlike the annelids,
however, mollusks lack body segmentation and their body shape is usually quite different,
as well.
The giant squid (Figure12.5), which until recently had not been observed alive in its adult
form, is one of the largest invertebrates. However, the colossal squid is even larger and can
grow up to 46 ft. (14 m) long. The smallest mollusks are snails that are microscopic in size.
Figure 12.5: The colossal squid, one of the largest invertebrates, here measuring 30 ft (9 m)
in length. ( 21 )