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The Silent Majority

Invertebrates defy nearly every generality. Of some 30 phyla representing 95 to 99 percent of the planet's animal species, the only thing they have in common is what they lack: a backbone. Otherwise, you would be hard pressed to find similarities among species as distinct as enormous moths, ethereal medusas, skittering scorpions, or stationary sponges. They may be spineless, but don't disregard these wildly diverse creatures: They're everywhere, and they have us seriously outnumbered...and outweighed. Scientists calculate that in the U.S., the combined weight of earthworms, insects, and spiders is 55 times greater than that of humans.

The major invertebrate phyla are:


Members of the phylum Porifera, ("pore-bearer") are commonly known as sponges, and approximately 5,000 have been described by scientists. Until the 18 th century, scientists mistook them for plants. Found worldwide in marine and fresh water environments, the sponges are notable for their unique feeding system. Specialized collar cells pump a current of water through pores into the sponge, which then filters out nutrients.

Though simple in design, sponges have strikingly bold colors and elaborate shapes. For instance, the aptly named orange puffball sponge, Tethya aurantia, is mostly spherical and orange to yellow in color. The delicately latticed Venus' flower basket sponge, Euplectella aspergillum, often shelters pairs of shrimp and as a result was once a symbol of marital fidelity in Japanese weddings.


Cnidarians, whose best-known representatives include jellyfish, corals, and sea anemones are characterized by one thrilling trait: they possess stinging cells called cnida (Greek for stinging nettle), from which the phylum takes its name. Its 9,500 members may appear serene and waif-like, but these carnivorous creatures can swiftly sting or entangle prey and defend themselves ably when threatened. For example, the Australian sea wasp, Tripedalia cystophora, has enough venom to kill 60 people.

The joined exoskeletons of tiny cnidarians called polyps form coral reefs. Coral reefs provide habitat to more than one fourth of marine animal species including fish, anemones, cephalopods, crustaceans, and sponges. Coral reefs also benefit humans by protecting seashore communities from strong waves and storms. Silt runoff from farming, roads, and ocean-side construction smothers the polyps and destroys delicate reef ecosystems.

more about cnidarians

Flat Worms, Round Worms, Segmented Worms

Phylum Platyhelminthes ("flat worms") includes some of nature's biggest freeloaders. Two-thirds of the 13,000 species in this taxon are parasitic, including the ultimate moocher, the tapeworm. With no mouth or digestive system to speak of, tapeworms (Class Cestoda) simply absorb their host's nutrients. A specialized epidermis allows these parasitic worms to survive in the gastrointestinal tracts of vertebrates without being digested. If not treated, tapeworms can be deadly to their hosts.

Phylum Nematoda, the round worms, contains parasites common to humans and domestic animals, including the species that causes heartworm in dogs. The vast majority of species is free-living (not parasitic) and harmless to humans. Nematodes can be found in abundance in all terrestrial and aquatic environments and at climatic extremes from arctic waters to arid deserts. A small amount of dirt or a piece of rotting fruit can contain hundreds to thousands of nematodes. The phylum contains an estimated 10,000 to 500,000 species.

The most familiar and appreciated kind of worm to gardeners—and anyone who likes to play in the dirt—are earthworms, members of phylum Annelida, as are thousands of species of marine worms and leeches. Commonly known as segmented worms, a series of repeated ring-like structures form the annelid body. The body of the Giant Gippsland earthworm (Megascolides australis) is composed of 500 to 600 segments, and individuals can grow up to ten feet in length.

Earthworms enrich the soil by eating dirt, literally. To burrow, earthworms may simply wiggle through the ground, but compact earth requires them to swallow the soil. This is also how these animals obtain nutrients. Along with dirt, they swallow decaying plants, seeds, animal larva, and other organic material. As they digest, they grind and sift this material and eventually excrete it back into the soil as castings—tiny piles of digested dirt that dot the surface of the ground. The castings are full of nutrients that are released into the soil and absorbed by plants.


Many stars of the Zoo's Invertebrate Exhibit can be found in phylum Mollusca, including the cuttlefish and nautilus, members of class Cephalopoda. Cephalopods, squid and octopus in particular, are some of the most intelligent invertebrates, capable of learning quickly and retaining information. Phylum Mollusca, containing an estimated 100,000 species, consists largely of class Gastropoda, represented by familiar creatures like slugs and snails. Enjoyed by many people for chowder and pearls, members of class Bivalvia secrete shells divided into two parts that are held closed by powerful muscles.

Mollusks have variously modified shells, ranging from the large, whorled shell of the whelk to the thin vestigial plate embedded in the body of a slug. Conchology is the study of mollusk shells.

More about cephalopods


It is nearly impossible to go a day without encountering one of the million members of this prolific phylum. More than three-quarters of the world's known animal species are arthropods, inhabiting air, land, and water. The major classes are the insects, representing 90 percent of arthropods, the arachnids, which include spiders and scorpions, and the crustaceans, containing crabs, barnacles, and lobsters. Arthropods wear protective, jointed exoskeletons, which they occasionally molt to allow for growth and repair.

As the animal kingdom's largest phylum, arthropods constitute an abundant food source. Members of the order Insectovora, such as shrews, hedgehogs, and moles, feed primarily on insects and other invertebrates. Baleen whales gulp down swarms of krill, a shrimp-like crustacean and major component of the whales' diet. Birds commonly feed on spiders, but the world's largest spider, the Goliath bird-eating tarantula (Therophosa leblondi), turns the tables on avian predators. The tarantula will eat young birds on occasion, although its usual diet consists of frogs, beetles, or small snakes.


Represented by starfish, sea urchins, and their relatives, phylum Echinodermata ("spiny-skinned") exclusively inhabits marine environments and consists of about 6,500 species. If you have ever seen the underside of a starfish, you may have noticed rows of fuzzy projections radiating from its center. These tentacle-like structures, called tube feet, are common to echinoderms and are used for food-gathering, locomotion, and sensation. All echinoderms possess a five-point radial organization, with five sets of organs, five major arteries, five-point symmetry, and in some cases, five sets of appendages.

The fleshy, oblong sea cucumber is radially symmetrical at the organ level only. They are some of the gutsiest echinoderms, spewing out their organs when disturbed and regenerating them over several weeks in a process called evisceration.