Class Gastropoda

The largest and most diverse class of mollusks is Gastropoda, whose members are called gastropods (GAS-troh-PAHDZ). Most of the 40,000 species of gastropods, including snails, abalones, and conchs, have a single shell. Others, such as slugs and nudibranchs, have no shell at all.

Gastropods undergo a process called torsion during larval development. During torsion, the visceral mass twists around 180 degrees in relation to the head. This twisting brings the mantle cavity, gills, and anus to the front of the animal, as shown in Figure 35-4. Because of torsion, a gastropod can withdraw its head into its mantle cavity when threatened. Coiling of the shell is unrelated to torsion.

Wavelike muscular contractions of the foot move gastropods smoothly over surfaces. You can see these contractions if you look closely at the underside of a snail or slug as it crawls across a windowpane or the side of an aquarium.

Gastropods have an open circulatory system, meaning that the circulatory fluid, called hemolymph, does not remain entirely within vessels. Instead, it is collected from the gills or lungs, pumped through the heart, and released directly into spaces in the tissues. These fluid-filled spaces compose what is known as a hemocoel (HEE-moh-SEEL), or blood cavity From the hemocoel, the hemolymph returns via the gills or lungs to the heart.

figure 35-4

In a gastropod, such as this snail, the mantle cavity, anus, and gills are near the head as a result of torsion during development.

Stomach

Intestine

Mantle cavity

In a gastropod, such as this snail, the mantle cavity, anus, and gills are near the head as a result of torsion during development.

Stomach

Intestine

Mantle cavity

Anus

figure 35-5

The extensions on the back of this horned nudibranch, Hermissenda crassicornia, provide a large surface area for gas exchange.

figure 35-5

The extensions on the back of this horned nudibranch, Hermissenda crassicornia, provide a large surface area for gas exchange.

Snails

Snails are gastropods that live on land, in fresh water, and in the ocean. Two eyes at the end of delicate tentacles on the head help most snails locate food. If danger arises, the tentacles retract into the head. Aquatic snails respire through gills in the mantle cavity. In land snails, the mantle cavity acts as a modified lung that exchanges oxygen and carbon dioxide with the air. The thin membrane lining the mantle cavity must be kept moist to allow gases to diffuse through it. For this reason, land snails are most active when the air has a high moisture content. Snails survive dry periods by becoming inactive and retreating into their shells. They seal the opening to their shell with a mucous plug, which keeps them from drying out.

Other Gastropods

Slugs are terrestrial gastropods that look like snails without shells. Like land snails, slugs respire through the lining of their mantle cavity. They avoid drying out by hiding in moist, shady places by day and feeding at night.

Nudibranchs (NOO-di-BRANGKS), such as the one in Figure 35-5, are marine gastropods that lack shells. Nudibranch means "naked gill," which refers to the fact that gas exchange occurs across the entire body surface of these animals. The surface of most nudibranchs is covered with numerous ruffles or delicate, fingerlike extensions that increase the total area available for gas exchange.

Some gastropods show unusual adaptations of the foot. In pteropods, or "sea butterflies," for example, the foot is modified into a winglike flap that is used for swimming rather than crawling.

Gastropods show great diversity in their reproduction. Many species have separate sexes, but hermaphrodites are common among aquatic and terrestrial gastropods.

figure 35-6

The two valves that make up the hinged shell of a bivalve can separate, allowing water to circulate through the animal. Some bivalves, such as this scallop, of the genus Argopecten, have a row of eyes near the outer margin of each valve.

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