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Mollusks Getting to Know Crustaceans Segmented Worms and their Relatives


Crustaceans, mollusks, segmented worms, and their relatives are incredibly different from each other. They differ in the shapes and makeup of their bodies, methods of reproduction and development, and in the ways they go about their lives. But they are related to one another based on a set of unique characteristics or features that they all have in common.

To understand these features, it is useful to know what makes up the bodies of all living things. The tissues of animals, plants, and other organisms are made up of cells. Cells are tiny units of life that are often surrounded by a skin-like covering called a membrane and have a nucleus (NU-klee-us) inside. The nucleus is the part of the cell that controls growth and reproduction.

All living things are divided into two major groups based on the structure of their cells. The first group includes bacteria and their relatives. These organisms have cells with no membranes and no nuclei (NU-klee-eye; the plural of nucleus). All remaining groups of organisms, including plants and animals, have cells with a nucleus and a membrane. These organisms include animals both with and without backbones. Those with backbones are called vertebrates (VER-teh-brehts), while those without are called invertebrates (in-VER-teh-brehts).

None of the animals included in this volume have backbones, but that is not why they are related. Instead, they are grouped together because they have other special features not found in other animals. Most of the features that unite these animals are seen only during the development of their eggs.

A developing animal egg is called an embryo (EM-bree-yo). Each embryo starts out as a single cell. Embryos develop and grow as the cell divides, doubling first into two cells, then four, then eight, and so on. They continue to divide until they form a hollow, ball-like mass of cells. By this time the cells are arranged into three distinct layers. Each layer is destined to become specific tissues and organs of the adult animal's body.

Cell division and tissue development in the embryo occurs in regular, predictable patterns. The animals included in this book, along with insects, spiders, and their relatives, undergo a special pattern of egg development. For example, in the tissues that eventually become the digestive system, the mouth develops first. Invertebrate animal embryos with mouths that develop first are called protostomes (PRO-toe-stomz). Protostome is based on the Greek words protos, meaning "first," and stoma, or "mouth."


Protostomes were here long before the dinosaurs. Ancient protostomes with shells, or hard external skeletons called exoskeletons, were the most likely to become fossils. Fossils are the remains of plants, animals, and other organisms that lived a long time ago. The remains are found in rocks and other materials. The earliest relatives of arthropods (crustaceans, insects, arachnids, and relatives) probably lived in ancient seas more An illustration depicting what the ocean may have looked like during the Jurassic Period; present are ammonite (Titanites anguiformis) based on fossils from Portland, Dorset, England and ichthyosaurs (Stenopterygius sp.) based on fossils from Holzmanaden, Germany. (© Chase Studios, Inc./Photo Researchers, Inc.) than 600 million years ago. Based on fossil evidence, velvet worms and mollusks are between 570 to 580 million years old. The oldest fossils that are definitely identified as crustaceans are about 530 million years old. The oldest fossils of water bears and segmented worms belong to species that lived in the oceans and are about 520 million years old. The first protostomes to leave the sea, including scorpions, centipedes, and millipedes, did so about 415 million years ago. The first insect fossils are about 400 million years old.


The ancient ancestors of protostomes probably had soft, wormlike bodies similar to today's sipunculans (phylum Sipuncula) and echiurans (phylum Echiura). These animals burrow in mud and sand on the ocean floor. But most protostomes have tough exoskeletons that support and protect their bodies. Having an exoskeleton allows these species to get out of the mud and sand and live in a wider variety of habitats, including land.

Like segmented worms, the bodies of arthropods are made up of repeating segments. Unlike worms, the exoskeletons of arthropods are hard and include jointed antennae, mouthparts, and legs. The body segments of arthropods are joined together to form specialized body regions, like a head and body trunk. In some groups of arthropods, the body trunk is divided into a thorax and abdomen. In crustaceans, the head and thorax are joined as one body region.

Having a tough exoskeleton puts limits on flexibility and growth. Crustaceans and other arthropods have solved these problems by having jointed limbs and other appendages to allow flexibility and by molting. Molting is when the animal grows a new exoskeleton and sheds the old one. Water bears (phylum Tardigrada) and velvet worms (phylum Onychophora) also molt.

Like exoskeletons, the shells of mollusks also provide support and protection. A thick sheet of skin called the mantle covers their soft bodies. The mantle has special tissues that make the shell or shell plates, forming a hard, protective shelter. Instead of limbs, a mollusk uses a powerful, muscular foot to get around.


Protostomes fly, swim, burrow, forage, and hunt to find food and mates, to reproduce, and to defend themselves. Most Different types of skeletons: external (snail), hydrostatic (earthworm), and jointed (scorpion). (Illustration by Kristen Workman. Reproduced by permission.) species live alone and come together only to mate. But some, like bryozoans (phylum Ectoprocta), are permanently connected to their neighbors. They live in branching masses that sometimes resemble plants. In some species, groups of individual bryozoans have special jobs, such as gathering food or forming a skeleton-like frame to support the colony.

Defensive behavior

Some protostomes defend themselves by standing their ground and flashing bright colors, waving menacing claws, or displaying sharp fangs. Others avoid danger by retreating into burrows or running, jumping, flying, or swimming away. Still Garden snails secrete bad tasting "foam" as a defense against predators. (© Holt Studios, Int./Photo Researchers, Inc.) others rely on armored defenses and depend on hard, bristly, spiny, or slimy bodies to repel hungry predators. Some species smell or taste bad, while others spray or squirt nasty chemicals at their enemies. Octopuses, cuttlefishes, and squids release a cloud of inky fluid to confuse and hide from predators.

Many protostomes avoid danger by using camouflage to blend in with their backgrounds. Others resemble objects that predators do not find appetizing, such as rocks, leaves, sticks, or bird droppings. Many biting, stinging, bad-tasting, and foul-smelling species do not hide at all. They are brightly marked or distinctively colored as a warning to potential predators. Species that do not bite or sting sometimes copy, or mimic, the colors, patterns, and body shapes of those that do in order to fool predators.


Herbivores (HER-bih-vorz) prefer to eat living plants. For example, many snails living on land and in the ocean use their Orange mantis shrimp live in burrows and prey on small fish which they "spear" with their sharp claws. (© David Hall/Photo Researchers, Inc.) rough, tonguelike mouthparts, or radula (RAE-juh-luh), to scrape up living plant tissues. But the sea snail known as the cone shell (Conus geographus) is a carnivore (KAR-nih-vor). It uses the teeth on its radula to harpoon marine worms and fish. Omnivores (am-nih-vorz) eat both plants and animals. Crayfish, shrimps, and octopuses are all omnivores. They, and other species of protostomes, will eat whatever they find. Blood-feeding leeches even become scavengers if a blood meal is not available.

Many marine and freshwater protostomes are filter feeders. They use special feeding structures to strain out or trap tiny bits of plants, animals, and other food floating in the water, or that has settled on the bottom. For example, brachiopods (phylum Brachiopoda) and bryozoans (phylum Ectoprocta) have special organs ringed with sticky tentacles that help them to catch tiny bits of food carried in the water.


Temperature, amount of daylight, moon cycle, seasonal rainfall, and tidal patterns are all signals that trigger reproduction. The rhythm of these natural events helps to coordinate the production of eggs and sperm so that they are ready at the same time, increasing the likelihood of fertilization.

Most species require males and females to reproduce. Ocean-dwelling species usually release their eggs and sperm into the This is a female water flea carrying eggs within her body. (© Holt Studios/Nigel Cattlin/Photo Researchers, Inc.) water. But in others, the female keeps the eggs inside her body and takes in sperm that has been released in the water. In the majority of species, males and females actually come into contact and mate. Males place their sperm, or sperm packets, directly on or inside the female's body. Hermaphrodites (her-MAE-fro-daits) are individual organisms that are able to make both eggs and sperm. In some hermaphrodites, male and female organs are present at the same time. However, in others, the adults start out as males and then become females. Sometimes the eggs develop without fertilization. This type of reproduction is called parthenogenesis (PAR-thih-no-JEH-ne-sis).

A few protostomes reproduce without any sperm or eggs at all. Instead, pieces of their bodies simply break off and the missing parts grow back. This process is called budding. Some protostomes use more than one means of reproduction.


The protostome embryo develops rapidly into a fully functional juvenile, or larva (LAR-vuh). The plural of larva is larvae (LAR-vee). The larvae have greater mobility than the adults. They often spend part of their lives floating around on ocean currents with other plankton. Plankton includes plants, animals, and other mostly tiny organisms. It is during the larval stages that many protostomes, such as segmented worms, mollusks, and crustaceans, move and settle into new habitats. In some groups, the larvae resemble small adults and reach adulthood directly by simply growing larger and gaining the ability to reproduce. Larvae that do not resemble the adults at all develop indirectly. They must go through several distinct stages before they acquire adult features.


Protostomes have long provided people with food, medicine, materials, and services. For centuries, clams, mussels, oysters, snails, and other mollusks have been an important part of the human diet throughout the world. Crustaceans, such as shrimps, lobsters, and crabs are also important sources of food. Since medieval times, leeches (phylum Annelida) have been The burrowing activities of earthworms enrich the soil, providing water, plant material and other nutrients. (© Holt Studios/Bob Gibbons/Photo Researchers, Inc.) used in various medical treatments. Today they are used to keep blood flowing into tissues after surgery, or to obtain other substances that are useful for treating disease. Other species of protostomes produce chemicals that are used as painkillers or show promise for treating cancer. Seashells are worn as jewelry and are also used as materials for making crafts and furniture. The shells of the cowry, a sea snail, were once used as money in parts of Asia, Africa, and elsewhere.

Protostomes are especially valuable for the services they provide. The burrowing activities of earthworms enrich the soil by mixing it with plant materials and other nutrients. Mussels, oysters, and barnacles keep oceans, bays, and estuaries clean and clear by filtering out huge amounts of plant, animal, and other tissues. For example, the oysters in the Chesapeake Bay once filtered the entire volume of water in the bay every three to four days. But today, after years of harvesting, there are only enough oysters left to filter the entire bay once a year. As a result, the water quality of the bay is very poor.

Some protostomes are harmful or are considered pests. For example, mollusks sometimes have other organisms living inside their bodies. These organisms can be dangerous to people who eat the mollusks. Bryozoans, earthworms, marine worms, crustaceans, insects, and mollusks sometimes become pests when they are accidentally introduced into a habitat outside of their normal distribution. Their presence can cause major changes in natural environments and can harm native species by competing with them for food and space.


Habitat destruction is the number one threat to protostomes, followed by over-harvesting, pollution, and the introduction of non-native species. Yet, people depend on them for food, pollination, and healthy habitats. Many governments recognize the importance of protostomes and have created laws that preserve habitats, limit hunting and fishing, and protect against the introduction of potentially harmful species.

To identify species in need of protection, the World Conservation Union (IUCN) publishes a list of species threatened by extinction called the Red List. Extinct species have completely died out and will never again appear on Earth. The IUCN Over-harvesting is a threat to many species that humans eat, yet many people depend on them for food. (© Vanessa Vick/Photo Researchers, Inc.) places species in the categories Extinct, Extinct in the Wild, Critically Endangered, Endangered, Vulnerable, Lower Risk, Near Threatened, Data Deficient, or Least Concern. The 2004 Red List includes 3,835 species of protostomes. Unfortunately, scientists may never know just how many species need protection. Their habitats are disappearing so quickly that scientists do not have enough time to study all of the species before they and their habitats are gone and lost forever.



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Web sites:

2004 IUCN Red List of Threatened Species. http://www.iucnredlist.org/ (accessed on May 8, 2005).

BioKIDS: Critter Catalog. http://www.biokids.umich.edu/critters/index.html (accessed on May 8, 2005).

Crustacea.net. http://www.crustacea.net/index.htm (accessed on May 8, 2005).

Ecowatch. http://www.ento.csiro.au/Ecowatch/index.htm (accessed on May 8, 2005).

Endangered Species Program. U.S. Fish & Wildlife Service. http://endangered.fws.gov/ (accessed on May 8, 2005).

Invertebrate Zoology. http://www.austmus.gov.au/invertebrates/index.htm (accessed on May 8, 2005).

Tree of Life Web Project. http://tolweb.org/tree/phylogeny.html (accessed on May 8, 2005).

Additional topics

Animal Life ResourceMollusks, Crustaceans, and Related Species