Mollusks evolved shells

Mollusks (phylum Mollusca) range in size from snails only a millimeter high to giant squids more than 18 meters long— the largest known invertebrates. Mollusks underwent one of the most dramatic of animal evolutionary radiations, based on a unique body plan with three major structural components: a foot, a mantle, and a visceral mass. Animals that appear very different, such as snails, clams, and squids, are all built from these components (Figure 32.25).

The molluscan foot is a large, muscular structure that originally was both an organ of locomotion and a support for the internal organs. In the lineage leading to squids and octopuses, the foot was modified to form arms and tentacles borne on a head with complex sense organs. In other groups, such as clams, the foot was transformed into a burrowing organ. In some lineages the foot is greatly reduced.

The mantle is a fold of tissue that covers the visceral mass of internal organs. In many mollusks, the mantle extends beyond the visceral mass to form a mantle cavity. The mantle secretes the hard, calcarous skeleton typical of most mollusks. The gills, which are used for gas exchange and, in some

Radula Mollusks

Gastropods

Intestine Stomach

Head Radula Mouth Digestive gland

Gastropods

Intestine Gill Anus Siphon

Cephalic tentacle Head Mouth

Mantle

Mantle

Tentacle Intestine

Foot

Shell plates Anus Foot

Gills in mantle cavity

Mantle cavity

Shell

Heart

Stomach

Salivary gland

Foot

The radula is a unique molluscan feeding structure modified for scraping.

Bivalves

The radula is a unique molluscan feeding structure modified for scraping.

Mollusks Cephalopods Structure

Cephalopods

Beak Arms

Head Radula

Shell

Stomach

Mantle

Head Radula

Shell

Stomach

Mantle Cavity

Siphon Mantle Gill cavity

Heart Intestine

In cephalod mollusks, the foot is modified into arms and tentacles.

Siphon Mantle Gill cavity

Heart Intestine

Tentacle

In cephalod mollusks, the foot is modified into arms and tentacles.

32.25 Molluscan Body Plans The diverse modern mollusks are all variations on a general body plan that includes a foot, a mantle, and a visceral mass of internal organs.

species, for feeding, lie in this cavity. When the cilia on the gills beat, they create a flow of water over the gills. The tissue of the gills, which is highly vascularized (contains many blood vessels), takes up O2 from the water and releases CO2.

Mollusks have an open circulatory system that empties into large fluid-filled cavities, through which fluids move around the animal and deliver O2 to internal organs. Mollusks also developed a rasping feeding structure known as the radula. The radula was originally an organ for scraping algae from rocks, a function it retains in many living mollusks. However, in some mollusks, it has been modified into a drill or poison dart. In others, such as clams, it is absent.

Although individual components have been lost in some lineages, these three unique, shared derived characteristics— the foot, the mantle, and the visceral mass—lead zoologists to believe that all 95,000 species of mollusks have a common ancestor. A small sample of these species is shown in Figure 32.26.

MONOPLACOPHORANS. Monoplacophorans (class Mono-placophora) were the most abundant mollusks during the Cambrian period, 550 million years ago, but today there are only a few surviving species. Unlike all other living mol-lusks, the surviving monoplacophorans have respiratory organs, muscles, and excretory pores that are repeated over the length of the body. The respiratory organs are located in a large cavity under the shell, through which oxygen-bearing water circulates.

CHITONS. Chitons (class Polyplacophora) have multiple gills and shell plates, but the body is not truly segmented (Figure 32.26a). The chiton body is bilaterally symmetrical, and its internal organs, particularly the digestive and nervous systems, are relatively simple. The larvae of chitons are almost indistinguishable from those of annelids. Most chitons are marine herbivores that scrape algae from rocks with their sharp radulae. An adult chiton spends most of its life clinging tightly to rock surfaces with its large, muscular, mucus-covered foot. It moves slowly by means of rippling waves of muscular contraction in the foot. Fertilization in most chitons takes place in the water, but in a few species fertilization is internal and embryos are brooded within the body.

bivalves. One lineage of early mollusks developed a hinged, two-part shell that extended over the sides of the body as well as the top, giving rise to the bivalves (class Bivalvia), which include the familiar clams, oysters, scallops, and mussels (Figure 32.26b). Bivalves are largely sedentary and have greatly reduced heads. The foot is compressed, and in many clams, it is used for burrowing into mud and sand. Bivalves feed by taking in water through an opening called an incurrent siphon and extracting food from the water with their large gills, which are also the main sites of gas exchange. Water and gametes exit through the excur-rent siphon. Fertilization takes place in open water in most species.

gastropods. Another lineage of early mollusks gave rise to the gastropods (class Gastropoda), which include snails, whelks, limpets, slugs, abalones, and the often brilliantly ornamented nudibranchs. Gastropods, unlike bivalves, have one-piece shells. Most gastropods are motile, using the large foot to move slowly across the substratum or to burrow through it. Gastropods are the most species-rich and widely distributed of the molluscan classes (Figure 32.26c,d). Most species move by gliding on the muscular foot, but in a few species—the sea butterflies and hetero-pods—the foot is modified into a swimming organ with which the animal moves through open ocean waters. The only mollusks that live in terrestrial environments—land snails and slugs—are gastropods. In these terrestrial species, the mantle tissue is modified into a highly vascu-larized lung. Fertilization is internal in most species.

cephalopods. In one lineage of mollusks, the cephalo-pods (class Cephalopoda), the excurrent siphon became modified to allow the animal to control the water content of the mantle cavity. Ultimately, the modification of the mantle into a device for forcibly ejecting water from the cavity enabled these animals to move rapidly through the water. Furthermore, many early cephalopods had chambered shells into which gas could be secreted to adjust buoyancy. Together, these adaptations allow cephalopods to live in open water.

The cephalopods include the squids, octopuses, and nautiluses (Figure 32.26e, f). They first appeared about 600 million years ago, near the beginning of the Cambrian period. By the Ordovician period a wide variety of types were present. With their greatly enhanced mobility, some cephalopods, such as squids, became the major predators in the open waters of the Devonian oceans.They remain important marine predators today. Cephalopods capture and subdue their prey with their tentacles; octopuses also use their tentacles to move over the substratum. As is typical of active predators, cephalopods have a head with complex sensory organs, most notably eyes that are comparable to those of vertebrates in their ability to resolve images. The head is closely associated with a large, branched foot that bears the tentacles and a siphon. The large muscular mantle provides a solid external supporting structure. The gills hang in the mantle cavity. As is typical of behaviorally complex animals, many cephalo-pods have elaborate courtship behavior, which may involve striking color changes.

Octopus Bimaculoides
(e) Octopus bimaculoides

32.26 Diversity among the Mollusks (a) Chitons are common in the intertidal zones of the temperate zone coasts. (b) The giant clam of Indonesia is among the largest of the bivalve mollusks. (c) Slugs are gastropods that have lost their shells; this shell-less sea slug is very conspicuously colored. (d) Land snails are shelled, terrestrial gas tropods. (e) Cephalopods such as the octopus are active predators.

(f) The boundaries of its chambers are clearly visible on the outer surface of this shelled Nautilus, another cephalopod.

The earliest cephalopod shells were divided by partitions penetrated by tubes through which liquids could be moved. Nautiloids (genus Nautilus) are the only cephalopods with external chambered shells that survive today (Figure 32.26f).

Mollusks and brachiopods are among the lophotro-chozoans that evolved hard shells that help to protect them from predators and the physical environment. A sturdy outer covering is the main feature of the second protostomate lineage, the ecdysozoans—the subject of the next chapter.

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  • leigh
    What is a siphon in a mollusk?
    8 years ago
  • marjorie
    What are the unique shared derived characteristics of molluscans?
    7 years ago

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