• Protists are neither animals nor plants; they are free- living, one-celled organisms.
  • Amoeboid forms include naked amoebas, Foraminifera with calcium carbonate skeletons, and Radiolaria with silica skeletons.
  • Ciliates are elongate, and their outer body is covered with cilia, which are used for locomotion.
  • Flagellates also are elongate, but they are propelled by one or a few flagellae instead of by many cilia.
  • Members of the phylum Porifera possess structures consisting of groups of flagellated cells, which move water and food particles into open chambers.
  • The cnidarians are all built around a common, cup-shaped polyp body plan, which has a ring of tentacles and a digestive tract with one opening.
  • Cnidarians are divided into Hydrozoa, Scyphozoa (true jellyfish), and Anthozoa (corals and anemones).
  • Flatworms are truly bilaterally symmetrical, with anterior-posterior differentiation. They also have distinct organs.
  • Ribbon worms have a proboscis and a complete gut and are mobile carnivores that burrow through the sand.
  • Many nematodes live free in all marine environments, using longitudinal muscles that work antagonistically against a fluid-filled body with a rigid wall. Free-living nematodes may be carnivorous or plant eating; some consume organic matter from sediment.
  • Annelids are worms divided into segments, with a tubular gut from mouth to anus and three distinct embryonic tissue layers.
  • The annelids are divided into the Polychaeta, Oligochaeta, and Hirudinea.
  • Annelid locomotion depends on layers of longitudinal and circular muscles working against a rigid fluid, compartmentalized among many segments.
  • Annelids adopt a wide range of living positions and include free burrowers, infaunal tube dwellers, and epifaunal tube constructors. They may be carnivores, deposit feeders, and suspension feeders.
  • Peanut worms live in burrows in soft sediment and in rock crevices. They gather food by means of an introvert that has branched tentacles.
  • The members of the phylum Pogonophora are generally deep-sea species that lack a gut and depend on symbiotic bacteria.
  • Mollusks have a head-foot complex, a mantle that usually secretes a calcium carbonate shell, and a gill, suspended in a mantle cavity, which is used for respiration and commonly for suspension feeding.
  • Members of the class Bivalvia are distinguished by two symmetrical shells connected in a hinge region. The mantle secretes the shell, and a gill, or ctenidium, usually helps in respiration and collects phytoplankton on ciliated tracts.
  • Members of the class Gastropoda have a flattened foot, usually a cap-shaped or coiled shell, and a mouth apparatus known as a radula. They are characterized by a twisting of the body, known as torsion. Many species lack a shell.
  • Members of the class Polyplacophora (chitons) have a flattened foot, a radula, and eight dorsal articulated plates.
  • Members of the class Monoplacophora include simple animals with a cap-shaped shell, posterior gill, flattened foot, and radula.
  • Members of the class Cephalopoda are distinguished by elaborate nervous and muscular coordination, the presence of grasping arms, and a carnivorous feeding mode.
  • Members of the class Scaphopoda (tusk shells) have an elongate conical shell and live buried within the sediment, feeding on foraminiferans and other small animals.
  • Arthropods are characterized by an external cuticle of chitin as well as by segmentation and jointed appendages.
  • The Trilobita (subphylum Trilobitomorpha) are an extinct class whose members had relatively unspecialized appendages and a compact body.
  • The subphylum Chelicerata includes the horseshoe crabs, spiders, terrestrial scorpions, and pycnogonids (sea spiders). These animals are characterized by a first pair of movable claws and a division of the body into two general sections.
  • The subphylum Crustacea is the largest marine arthropod group. Its members are characterized by a head with two pairs of antennae, three pairs of mouthpart appendages, and a trunk with several specialized appendages. The trunk is sometimes divided into a thorax and a posterior abdomen.
  • The lophophorate phyla include the Bryozoa, Brachiopoda, and Phoronida. They are united by the presence of a looped feeding and respiring structure known as a lophophore.
  • Bryozoans are abundant on hard surfaces and consist of colonies of small (<1 mm in width) individuals known as zooids.
  • Bryozoan colonies can occur as sheets, erect colonies, or units connected by runners, known as stolons.
  • Brachiopods have two shells and use a pedicle to attach to the bottom; a lophophore allows them to feed on suspended matter.
  • Phoronids are wormlike, with a lophophore that protrudes above the substratum.
  • Echinoderms are exclusively marine and have an outer skin that encloses a skeleton of interlocking ossicles.
  • Both locomotion and feeding in echinoderms are based on the water vascular system, which uses water pressure to operate many tube feet.
  • Sea stars (Asteroidea) are usually carnivorous, and many feed by extruding the stomach and digesting prey, mostly outside the body.
  • Echinoids (Echinoidea) are usually covered with spines and may live on the surface, using the teeth of a structure called an Aristotle’s lantern to scrape algae, or burrowing in the sand, feeding on sediment or suspended matter.
  • Sea lilies and feather stars (Class Crinoidea) are characterized by a cup-shaped body, with upward- reaching arms that catch zooplankton on specialized tube feet.
  • Sea cucumbers (Class Holothuroidea) are tubular and have a crown of tentacles, which either feed on sediment or are directed upward to feed on zooplankton.
  • Brittle stars and basket stars have a central disk and distinct, separate, flexible arms that move the animal along the bottom without the use of tube feet.
  • Sea squirts are barrel-shaped animals that filter water through a mucus sheet. A tadpole-shaped larva relates them to the vertebrates.
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