Characteristics Of Mollusks

The phylum Mollusca is a diverse group of more than 112,000 species. Among animals, only the phylum Arthropoda has more species. Some mollusks are sedentary filter feeders, while others are fast-moving predators with complex nervous systems.

Mollusks are among several phyla of animals known as coelomates. Coelomates are so named because they have a true coelom, a hollow, fluid-filled cavity that is completely surrounded by mesoderm. Coelomates differ from pseudocoelomates, such as roundworms, which have a body cavity lined by mesoderm on the outside and endoderm on the inside.

A coelom has several advantages over a pseudocoelom. With a coelom, the muscles of the body wall are separated from those of the gut. Therefore, the body wall muscles can contract without hindering the movement of food through the gut. A coelom also provides a space where the circulatory system can transport blood without interference from other internal organs. The coelomate body plan is shared by annelids, which are discussed in the second half of this chapter, and by three other major phyla of animals: arthropods, echinoderms, and chor-dates, which include humans.

Another feature that is shared by most aquatic mollusks and annelids is a larval stage of development called a trochophore (TRAHK-oh-FAWR), illustrated in Figure 35-1. In some species, the trochophore hatches from the egg case and exists as a free-swimming larva. Cilia on the surface of a free-swimming trochophore propel the larva through the water and draw food into its mouth. As free-swimming trochophores are carried by ocean currents and tides, they contribute to the dispersal of their species. The presence of a trochophore in mollusks and annelids suggests that these two groups of animals may have evolved from a common ancestor.

objectives

• Describe the key characteristics of mollusks.

• Describe the body plan of mollusks.

• Name the characteristics of three major classes of mollusks.

• Compare the body plans of gastropods, bivalves, and cephalopods.

vocabulary trochophore visceral mass mantle mantle cavity ganglion radula gastropod hemolymph hemocoel bivalve incurrent siphon excurrent siphon cephalopod figure 35-1

A trochophore is a larva that develops from the fertilized egg of most mollusks and annelids. Cilia at both ends and in the middle propel free-swimming trochophores through the water.

figure 35-1

A trochophore is a larva that develops from the fertilized egg of most mollusks and annelids. Cilia at both ends and in the middle propel free-swimming trochophores through the water.

figure 35-2

In the basic body plan of a mollusk, the body is divided into the head-foot and the visceral mass, which contains the internal organs. Covering the visceral mass is the mantle, which secretes the shell.

Visceral mass Head-foot figure 35-2

In the basic body plan of a mollusk, the body is divided into the head-foot and the visceral mass, which contains the internal organs. Covering the visceral mass is the mantle, which secretes the shell.

Visceral mass Head-foot

Squid Physiology

figure 35-3

Inside the mouth (a), many mollusks have a radula, a band of tissue covered with teeth that can scrape food from other surfaces. The SEM in (b) shows the sharp edges of these teeth (600x).

Radula Mouth figure 35-3

Inside the mouth (a), many mollusks have a radula, a band of tissue covered with teeth that can scrape food from other surfaces. The SEM in (b) shows the sharp edges of these teeth (600x).

Radula Mouth

Teeth

Teeth

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