From the list below, select the letter that corresponds to the first letter of the word that you wish to look for.
abdomen - Region of the body furthest from the mouth. In insects, the third body region behind the head and thorax.
ambulacra - Row of tube feet of an echinoderm.
anus - End of the digestive tract, or gut, through which waste products of digestion are excreted, as distinct from the mouth.
benthic - Organisms that live on the bottom of the ocean are called benthic organisms. They are not free-floating like pelagic organisms are.
biramous - Arthropod appendages that are biramous have two branches, an outer branch and an inner branch. These branches may have separate functions; in crustaceans, for instance, the inner branch of a leg is used for walking, while the outer branch may be paddle-shaped or feathery and often functions as a gill. Contrast with uniramous.
blood - Fluid which circulates throughout the body, distributing nutrients, and oxygen as well in many animals.
book lung - A set of soft overlapping flaps, covered up by a plate on the abdomen, through which oxygen is taken up and carbon dioxide given off. Characteristic of many terrestrial arachnids such as scorpions and spiders.
brain - Collection of nerve cells usually located at the anterior end of an animal, when present at all. The nerves coordinate information gathered by sense organs, locomotion, and most internal body activities.
cephalon - In trilobites, the head shield bearing the eyes, antennae, and mouth. More info?
chaetae - Stiff bristles characteristic of annelids.
chela - The claw of an arthropod.
chelicera - The first pair of appendages of a chelicerate arthropod. Originally a short clawed appendage, the chelicerae of many arachnids are highly modified for feeding; in spiders, for instance, they are modified into poisonous fangs.
clitellum - In annelids, a swelling of the body towards the head of the animal, where the gonads are located. Both oligochaetes and leeches have a clitellum.
cnidocyst - The "stinging cell" of a cnidarian.
coelom - Fluid-filled cavity within the body of an animal; usually refers to a cavity lined with specialized tissue peritoneum in which the gut is suspended. The structure and development of the coelom is an important character for recognizing major groups of animals.
compound eye - Found in many but not all arthropods, a compound eye is composed of a large number of small, closely packed simple eyes (ommatidia), each with its own lens and nerve receptors.
cuticle - In animals, a multilayered, extracellular, external body covering, usually composed of fibrous molecules such as chitin or collagen, and sometimes strengthened by the deposition of minerals such as calcium carbonate.
ectoderm - The outer basic layer of tissue in those animals with true tissues. In vertebrates, for instance, the embryonic ectoderm differentiates into the skin and also the nervous system.
endoderm - The innermost basic layer of tissue in those animals with true tissues. Forms the gut and its derivatives: in vertebrates, these include the liver, trachea, and lungs.
epidermis - The outermost layer of skin.
epithelium - Layer of cells which lines a body cavity; cells may be ciliated or unciliated, and may be squamous (flat, scale-shaped), cuboidal (cube-shaped), or columnar (column-shaped). Your stomach and cheeks are lined with epithelium.
esophagus - That portion of the gut which connects the pharynx to the stomach.
gastrodermis - In cnidarians, the endodermis which lines the gut cavity. The term is often used instead of endodermis since cnidarians only have two tissue layers instead of three.
gill - In aquatic animals, highly vascularized tissues with large surface area; these are extended out of the body and into the surrounding water for gas exchange.
gill arches - Stiffenings which support the flesh between the gill slits of chordates. In most vertebrates, the first gill arches have been modified to form the jaw, and in tetrapods, the inner ear bones.
gill slit - A slitlike or porelike opening connecting the pharynx of a chordate with the outside of the body. Gill slits may contain the gills and be used for gas exchange, as in most fish, but may also be used for filter-feeding, or may be highly modified in land-dwelling vertebrates.
gnathobase - The expanded and hardened base of the appendage of many arthropods, notably trilobites, crustaceans, and marine cheliceramorphs. Used to macerate food items before ingestion.
gut (enteron) - Body cavity formed between the mouth and anus in which food is digested and nutrients absorbed; it consists of the mouth, pharynx, esophagus, stomach, intestine, and anus, though some animals do not have all these regions.
head - That part of the body at the "front" end, where the brain, mouth, and most sensory organs are located.
heart - Muscular pump which circulates the blood.
intestine - The portion of the digestive tract between the stomach and anus; it is the region where most of the nutrients and absorbed.
jaw - Often loosely applied to any movable, toothed structures at or near the mouth of an animal, such as the scolecodonts of annelids. In vertebrates, the jaw is derived from the first gill arch.
jointed - When stiff body parts are connected by a soft flexible region, the body is said to be jointed.
librigenae - The "free cheeks"; separate, detachable portions of the trilobite cephalon.
lophophore - Complex ring of hollow tentacles used a feeding organ. The tentacles are covered by cilia, which generate a current to bring food particles into the mouth. The structure is only found in the brachiopods, phoronids, and bryozoans.
mesoderm - In animals with three tissue layers (i.e. all except sponges and cnidarians), the middle layer of tissue, between the ectoderm and the endoderm. In vertebrates, for instance, the mesoderm forms the skeleton, muscles, heart, spleen, and many other internal organs.
mesogloea - Jellylike material between the outer ectoderm and the inner endoderm of cnidarians. May be very thin or may form a thick layer (as in many jellyfish)
mouth - Front opening of the digestive tract, into which food is taken for digestion. In flatworms, the mouth is the only opening into the digestive cavity, and is located on the "belly" of the worm.
mucus - Sticky secretion used variously for locomotion, lubication, or protection from foreign particles.
muscle - Bundle of contractile cells which allow animals to move. Muscles must act against a skeleton to effect movement.
myotome - Segment of the body formed by a region of muscle. The myotomes are an important feature for recognizing early chordates.
nematocyst - Older name for a cnidocyst.
neuron - A specialized cell that can react to stimuli and transmit impulses. A neuron consists of a body which contains the nucleus; dendrites, which are short branches off the body that receive incoming impulses; and a long axon which carries impulses away from the body and to the next neuron.
nerve - A bundle of neurons, or nerve cells. More properly, it is a bundle of axons.
nerve cord - Primary bundle of nerves in chordates, which connects the brain to the major muscles and organs of the body.
notochord - Characteristic of chordates, the notochord is a stiff rod of tissue along the back of the body. In vertebrates, the backbone is deposited around the notochord and nerve cord.
organ - Collection of tissues which performs a particular function or set of functions in an animal's body. The heart, brain, and skin are three organs found in most animals. Organs are composed of tissues, and may be organized into larger organ systems.
operculum - Small disk-like cover or lid of an aperture, commonly found in cheilostomes.
organ system - Collection of organs which have related roles in an organism's functioning. The nervous system, vascular system, and muscle system are all organ systems.
osculum - The main opening through which filtered water is discharged. Found in sponges.
papilla(e) - Cellular outgrowths. These look like little bumps or fingers on the surface of cells.
parapodia - A sort of "false foot" formed by extension of the body cavity. Polychaetes and some insect larvae have parapodia in addition to their legs, and these provide extra help in locomotion.
pedipalps - The second pair of appendages of cheliceromorphs. In many arachnids, such as spiders, the pedipalps are enlarged in the male and used for copulation.
pharyngeal slits - Characteristic of chordates, pharyngeal slits are openings through which water is taken into the pharynx, or throat. In primitive chordates the pharyngeal slits are used to strain water and filter out food particles; in fishes they are modified for respiration. Most terrestrial vertebrates have pharyngeal slits only in the embryonic stage.
pharynx - Cavity in the digestive tract just past the mouth itself. May be muscularized for sucking or swallowing in various animals.
pleurae - In trilobites and other arthropods, pleurae are elongated flat outgrowths from each body segment, that overlie and protect the appendages.
pore - Any opening into or through a tissue or body structure.
proboscis - Elongated organ, usually associated with the mouth. The proboscis is an important feeding appendage in echiurans.
pygidium - In trilobites, the posterior division of the body, formed by fusion of the telson with one or more posterior pleurae.
segmentation - In many animals, the body is divided into repeated subunits called segments, such as those in centipedes, insects, and annelids. Segmentation is the state of having or developing a body plan in this way.
septum - Partition which divides up a larger region into smaller ones, such as in the central body cavity of some anthozoa.
siphon - Opening in molluscs or in urochordates which draws water into the body cavity. In many molluscs, the siphon may be used to expel water forcibly, providing a means of propulsion.
skeleton - Support structure in animals, against which the force of muscles acts. Vertebrates have a skeleton of bone or cartilage; arthropods have one made of chitin; while many other invertebrates use a hydrostatic skeleton, which is merely an incompressible fluid-filled region of their body.
spicule - Crystalline or mineral deposits found in sponges, sea cucumbers, or urochordates. They are structural components in many sponges, and may serve a protective function in other organisms.
spiracle - In insects and some other terrestrial arthropods, a small opening through which air is taken into the tracheae. Insects have several spiracles, arranged along the sides of the abdomen.
spongocoel - Central body cavity of sponges.
telson - The last segment of the abdomen in many arthropods. May be flat and paddlelike, buttonlike, or long and spiny, as in the horseshoe crabs.
tentacles - Appendages which are flexible, because they have no rigid skeleton. Cnidarians and molluscs are two kinds of orgnaisms which may have tentacles.
thorax - In insects, the second body region, between the head and thorax. It is the region where the legs and wings are attached.
tissue - A group of cells with a specific function in the body of an organism. Lung tissue, vascular tissues, and muscle tissue are all kinds of tissues found in some animals. Tissues are usually composed of nearly identical cells, and are often organized into larger units called organs.
tracheae - Internal tubes through which air is taken for respiration. Vertebrates with lungs have a single trachea carrying air to the lungs, while insects and some other land-living arthropods have a complex network of tracheae carrying air from the spiracles to all parts of the body.
tube feet - Extensions of the water-vascular system of echinoderms, protruding from the body and often ending in suckers. May be used for locomotion and/or for maintaining a tight grip on prey or on the bottom.
tubercle - Any small rounded protrusion. In pycnogonids and some cheliceramorph arthropods, the central eyes are carried on a tubercle.
uniramious - Among arthropods, uniramous refers to appendages that have only one branch. Insects, centipedes and millipedes, and their relatives are uniramous arthropods; land-living chelicerates such as scorpions, spiders, and mites are also uniramous but probably descended from ancestors with biramous appendages. Contrast with biramous.
vascular - Refers to a network of tubes which distribute nutrients and remove wates from the tissues of the body. Large multicellular animals must rely on a vascular system to keep their cells nourished and alive.
vertebra - A component of the vertebral column, or backbone, found in vertebrates.
zooaria - The colony of bryozoan animals.
zooecia (zooecium singular) - Bryozoan living chambers constructed by a colony of zooids. The zooecium is he living chamber constructed by one bryozoan individual.
zooid - Individual animal or member in a bryozoan colony.
zooxanthellae - Symbiotic dinoflagellates in the genus Symbiodinium that live in the tissues of a number of marine invertebrates and protists, notably in many foraminiferans, cnidarians, and some mollusks.