Bugs, Mollusks & Other Invertebrates

When it comes to creepy-crawlies, arachnids are some of the first critters that might come to mind. This arthropod group includes spiders, daddy longlegs, and scorpions, among other creatures that many of us would prefer to avoid.

Arthropoda is the largest phylum in the animal kingdom; about 84 percent of all known species of animals fall into this category. Lobsters, crabs, spiders, mites, insects, centipedes, and millipedes are all arthropods.

Aschelminth, phylum name Aschelminthes, or Nemathelminthes, a name referring to an obsolete phylum of wormlike invertebrates, mostly of microscopic size. Previously, phylum Aschelminthes included seven diverse classes of animals: Nematoda (or Nemata), Rotifera, Acanthocephala, Gastrotricha, Kinorhyncha (or Echinodera), Nematomorpha, and Gnathostomulida. (According to some authorities, Gnathostomulida was replaced by Priapula in this list.) At present, each of these classes, including Priapula, has been reclassified as a separate phylum.

The phylum Cnidaria is made up of more than 9,000 living species, mostly marine animals. Corals, jellyfish, sea anemones, and sea fans are all cnidarians.

Comb jellies are any of the numerous marine invertebrates that constitute the phylum Ctenophora.

Crustaceans are a group of invertebrate animals consisting of some 45,000 species worldwide. Crabs, lobsters, and shrimps are all counted among the subphylum Crustacea. Crustaceans are found primarily in water, as noted by Sebastian the crab in Disney’s “The Little Mermaid” (1989).

When people talk about “insects,” they often mean pests or disease carriers such as bedbugs, houseflies, beetles, mosquitoes, fleas, and hornets. To be fair, though, many insects are beneficial to humans, albeit sometimes indirectly: they pollinate plants, produce useful substances, control pest insects, act as scavengers, and serve as food for other animals. Some insects, such as butterflies, are beautiful to behold, and insects such as ladybugs flaunt eye-catching colors and patterns.

Mollusk, also spelled mollusc, any soft-bodied invertebrate of the phylum Mollusca, usually wholly or partly enclosed in a calcium carbonate shell secreted by a soft mantle covering the body. Along with the insects and vertebrates, it is one of the most diverse groups in the animal kingdom, with nearly 100,000 (possibly as many as 150,000) described species. Each group includes an ecologically and structurally immense variety of forms: the shell-less Caudofoveata; the narrow-footed gliders (Solenogastres); the serially valved chitons (Placophora or Polyplacophora); the cap-shaped neopilinids (Monoplacophora); the limpets, snails, and slugs (Gastropoda); the clams, mussels, scallops, oysters, shipworms, and cockles (Bivalvia); the tubiform to barrel-shaped tusk shells (Scaphopoda); and the nautiluses, cuttlefishes, squids, and octopuses (Cephalopoda).

Moss animal, also called bryozoan, any member of the phylum Bryozoa (also called Polyzoa or Ectoprocta), in which there are about 5,000 extant species. Another 15,000 species are known only from fossils. As with brachiopods and phoronids, bryozoans possess a peculiar ring of ciliated tentacles, called a lophophore, for collecting food particles suspended in the water. The bryozoans are a widely distributed, aquatic, invertebrate group of animals whose members form colonies composed of numerous connected units called zooids (hence the term Polyzoa, which means “many animals”).

Echinoderm, any of a variety of invertebrate marine animals belonging to the phylum Echinodermata, characterized by a hard, spiny covering or skin. Beginning with the dawn of the Cambrian Period (542 million to 488 million years ago), echinoderms have a rich fossil history and are well represented...

Sponge, any of the primitive multicellular aquatic animals that constitute the phylum Porifera. They number approximately 5,000 described species and inhabit all seas, where they occur attached to surfaces from the intertidal zone to depths of 8,500 metres (29,000 feet) or more. The members of one family, the Spongillidae, are found in fresh water; however, 98 percent of all sponge species are marine. Adult sponges lack a definite nervous system and musculature and do not show conspicuous movements of body parts.

Annelid, phylum name Annelida, also called segmented worm, any member of a phylum of invertebrate animals that are characterized by the possession of a body cavity (or coelom), movable bristles (or setae), and a body divided into segments by transverse rings, or annulations, from which they take their name. The coelom is reduced in leeches, and setae are lacking a few specialized forms, including leeches.