Mollusks

In general, mollusks are more appealing than other invertebrates. Instead of slithering or crawling around, they live in cute little shells. The mollusks include the gastropods (snails), pelecypoda (clams), and cephalopods (squids and octopuses). The study of mollusks demonstrates the relationship between physiology and lifestyle. For example, many do not need efficient circulatory systems because they are slow moving.

The gastropods include sea snails, sea slugs, land snails, and land slugs. There are up to 80,000 species. Gastropod means stomach foot. Snails have a shell while slugs have no shell or a very small shell. Because they move in search of food, gastropods have a head and tentacles with eyes. They also have an olfactory system. Gastropods have a central and peripheral nervous system. In snails, the stomach became the foot. Gastropods have gills or lungs for gas exchange with the atmosphere. Their circulatory systems are incomplete. They do not need an efficient circulatory system because they are so slow moving (Figure 8‑24).

Figure 8‑24. The anatomy of a common air-breathing land snail. Credit: Ai2/Jeff Dahl. Used here per CC BY-SA 4.0.

The Pelecypoda are the bivalves (clams). They are filter feeders that suck water into their gills, where they filter out nutrients and perform gas exchange. These nonmoving animals have little need for a sensory structure or even a front end. Only a few species have eyes. They have less need of a nervous system than other mollusks so it is much less complex. Bivalves have a mantle that forms the shell (Figure 8‑25). They have a set of gills, but their respiratory and circulation system is less developed than other gastropods. As with snails and slugs, they have a muscular foot, which allows them to burrow into the sea floor. They also have a muscle that opens and closes the shell. They have a mouth, stomach, intestine, and anus, through which they process food.

Figure 8‑25. Giant clam (Tridacna gigas) Credit: Drow male. Used here per CC BY-SA 4.0.

The Pelecypoda are the bivalves (clams). They are filter feeders that suck water into their gills, where they filter out nutrients and perform gas exchange. These nonmoving animals have little need for a sensory structure or even a front end. Only a few species have eyes. They have less need of a nervous system than other mollusks so it is much less complex. Bivalves have a mantle that forms the shell They have a set of gills, but their respiratory and circulation system is less developed than other gastropods. As with snails and slugs, they have a muscular foot, which allows them to burrow into the sea floor. They also have a muscle that opens and closes the shell. They have a mouth, stomach, intestine, and anus, through which they process food. The largest bivalve is appropriately named the giant clam (Figure 8-25).

Figure 8-26. Caribbean reef squid (Sepioteuthis sepioidea). Credit: Wikipedia. Nick Hobgood.

The cephalopods include squids (Figure 8-26) and octopi. The head of the squid has the same origin as the foot of the gastropod; thus, their name means head foot. The squids are hunters, and in order to become efficient hunters, they evolved eyes and tentacles for grasping prey. Like the other mollusks, the cephalopods have gills. Unlike the others, they have closed circulatory system, which is required because they are fast movers. The octopus, another cephalopod, has a large brain, approximately the size of a dog's brain. However, the structure of the brain is quite different from the vertebrate brain. Almost all (90%) of the brain is in the eye structures and in the tentacles. Squids have an eye that is like a vertebrate eye in overall structure although many of the vertebrate eye capabilities are missing.

As described in section 8-4, mollusks have the earliest preserved fossils of bilaterians. Kimbrella quadrata (558 Ma) might have been a stem group mollusk. Halkeria (535 to 530 Ma also might have been a stem mollusk. They were small slugs with a shell made of 2000 plates on the back. Scientists have speculated on the characteristics of the ancestral mollusk (Figure 8‑27).

Figure 8‑27. The hypothetical ancestral mollusk. Credit: KDS444. Used here per CC BY-SA 4.0.

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Lined chiton from Whidbey Island, Washington. Credit: Kurt L. Onthank. Used here per CC BY-SA 3.0.

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