Animal Life

Cephalopoda (Nautilids, octopods, cuttlefishes, squids, and relatives) Phylum Mollusca Number of families About 45 Thumbnail description Mollusks bearing a radula (toothed tongue) and well-developed heads; mouth characterized by a dorsoventral pair of horny jaws known as beaks and encircled by the bases of 8-ca. 60 grasping appendages; single pair of lateral, image-forming eyes; well-developed brain […]


Class Polychaeta

Number of families 86 Thumbnail description Segmented worms with numerous bristles and one pair of parapodia per segment Photo: Bristle or fire worms (Chloeia sp.) primarily come out at night and are scavangers. Evolution and systematics Polychaetes do not fossilize very well as they are soft-bodied animals. There are few fossil records from an entire […]

Class Myzostomida

Number of families 8 Thumbnail description Minute, soft-bodied marine worms associated with echinoderms Photo: A dorsal view of Myzostoma polycyclus, collected on a crinoid from Madagascar. Evolution and systematics The name “myzostomid” comes from the Greek myzo, meaning “to suck,” and stoma, meaning “mouth.” Leuckart described the first myzostomid, Myzostoma parasiticum, in 1827 and 1836. […]

Class Oligochaeta

Oligochaeta (Earthworms) Subphylum Clitellata Number of families 17 Thumbnail description Terrestrial worms that typically dwell in soil and that are characterized by a “tube within a tube” construction, with an outer muscular body wall surrounding a digestive tract that begins with the mouth in the first segment and ends with the anus in the last […]

Class Hirudinea

Hirudinea (Leeches) Number of families 14 Thumbnail description Annelids possessing a caudal sucker used for attachment to surfaces, and a fixed number of body segments with subdivided annuli (ring-like structures); best known for their blood-sucking members and their use in medicine Photo: Newborn brood of aquatic leaf leeches. Evolution and systematics Leeches are completely soft-bodied […]

Class Pogonophora

Pogonophora (Beard worms) Number of families 13 Thumbnail description Tube-living marine worms nourished by internal chemoautotrophic bacteria. Photo: Ridgeia piscesae at Middle Valley hydrothermal site in the northeast Pacific Ocean, photographed through the porthole of Alvin, the Navy-owned deep submergence vehicle. The tubes are 0.20-0.39 in (5-10 mm) in diameter. Evolution and systematics The name […]

Phylum Vestimentifera

Vestimentifera (Hydrothermal vent and cold seep worms) Number of families 8 Thumbnail description Segmented worms that have an unusual anatomy and rely on symbiotic bacteria for nutrition. They are nearly always found in deep waters, some as members of hydrothermal vent communities and others found in association with reducing sediments such as “cold seeps” Photo: […]

Phylum Sipuncula

Sipuncula (Peanut worms) Number of families 6 Thumbnail description Unsegmented marine worm-like animals with a body divided into a trunk and retractable introvert Photo: Peanut worm (Phascolosoma sp.) with partially extended introvert. Evolution and systematics No unambiguous fossil Sipuncula are currently known. Ot-toia prolifica from the Burgess Shale has been proposed as a fossil sipunculan, […]

Phylum Echiura

Echiura (Echiurans) Number of families 5 Thumbnail description Worm-like invertebrates with non-segmented, bilaterally symmetrical bodies Photo: A spoon worm (Bonellia) in Western Australia, with its forked tongue fully extended. Evolution and systematics As the echiurans’ body has no large hard parts, fossils of these animals are rare. There are two fossils for this group: a […]