Zoology: Invertebrates

Abdomen

The rear section of an animal's body that contains digestive and reproductive organs.

Acoelomate

An animal with no body cavity between the digestive tract and body wall.

Bilateral symmetry

Body plan where the left and right sides are mirror images.

Body cavity (Coelom)

A fluid-filled space that holds internal organs.

Cephalization

Concentration of sense organs and nerve cells at the head end of the body.

Closed circulatory system

A circulatory system where blood stays inside vessels.

Coelomate

An animal that has a true body cavity fully lined with mesoderm.

Diffusion

Movement of molecules from high to low concentration without energy use.

Exoskeleton

A hard, outer covering that protects and supports the body.

Filter feeding

Feeding method where an animal strains tiny food particles from water.

Gastrovascular cavity

A simple internal cavity with one opening for both food intake and waste release.

Hermaphrodite

An organism that has both male and female reproductive organs.

Hydrostatic skeleton

A fluid-filled internal cavity that muscles push against to create movement.

Invertebrate

An animal that does not have a backbone.

Motile

Capable of movement.

Open circulatory system

A system where blood flows freely in body spaces instead of closed vessels.

Protostome

An organism whose mouth develops before the anus during embryonic development.

Radial symmetry

Body parts arranged around a central point.

Segmentation

Division of the body into repeating sections.

Sessile

Permanently attached and unable to move.

Amoebocyte

Mobile sponge cell that moves nutrients and makes spicules.

Budding

Asexual reproduction where a new sponge grows from the parent's body.

Choanocyte (Collar cell)

Sponge cell with a flagellum that creates water flow and traps food.

Osculum

Large opening at the top of a sponge where water exits.

Ostia (Pores)

Small openings in a sponge that allow water to enter.

Regeneration

The ability to regrow missing body parts.

Spicules

Hard, needle-like structures that give a sponge support.

Spongin

Soft, flexible protein fibers that provide sponge structure.

Cnidarian

Aquatic animals with stinging cells and radial symmetry.

Cnidocyte

Specialized stinging cell used to capture prey.

Coral reef

Large underwater structure made by colonies of coral polyps secreting calcium carbonate.

Gastrodermis

Inner tissue layer that lines the gastrovascular cavity.

Medusa

Bell-shaped, free-swimming body form of cnidarians.

Mesoglea

Jelly-like layer between the inner and outer tissue layers.

Nematocyst

Coiled, harpoon-like capsule inside a cnidocyte that delivers a sting.

Nerve net

Simple web of nerve cells that control movement without a brain.

Polyp

Vase-shaped, sessile body form of cnidarians.

Tentacle

Flexible arm-like structure used for sensing and capturing prey.

Eyespot

Light-sensitive organ that helps detect direction of light.

Flame cell

Specialized cell that removes waste and regulates water balance.

Flatworm (Platyhelminth)

Simple, flat-bodied worm with bilateral symmetry.

Host

Organism that a parasite lives on or inside.

Intermediate host

Host that supports an immature or non-reproductive form of a parasite.

Parasitic

Living in or on another organism and harming it.

Pharynx

Muscular tube used to suck in food.

Planarian

Free-living flatworm known for regeneration ability.

Annelid

Segmented worm with a true coelom and closed circulatory system.

Aortic arches

Series of muscular 'hearts' that pump blood through an earthworm.

Clitellum

Thickened band that secretes mucus to form an egg cocoon.

Crop

Storage chamber in an earthworm's digestive system.

Dorsal

The top or back side of an organism.

Gizzard

Muscular organ that grinds food before digestion.

Nephridium

Excretory organ that removes liquid waste from the body.

Segment

Repeated body unit containing similar organs.

Setae

Bristle-like structures used for movement and traction.

Ventral

The bottom or belly side of an organism.

Bivalve

Mollusk with two shells connected by a hinge (like a clam).

Cephalopod

'Head-footed' mollusk, such as squid or octopus, with tentacles and a developed brain.

Chromatophore

Pigment cell that allows color change for camouflage or communication.

Foot

Muscular structure used for movement or digging.

Gills

Thin structures that exchange oxygen and carbon dioxide with water.

Mantle

Tissue layer that secretes the shell and covers internal organs.

Radula

Tongue-like organ covered in teeth used to scrape or cut food.

Shell

Hard, protective external covering secreted by the mantle.

Siphon

Tube-like structure that moves water for jet propulsion or feeding.

Visceral mass

Central region containing internal organs.

Antenna (Antennae)

Sensory appendage used to detect touch, smell, or vibration.

Arthropod

Animal with jointed appendages, segmented body, and exoskeleton.

Cephalothorax

Fused head and thorax found in spiders and crustaceans.

Compound eye

Eye made of many small lenses for detecting movement.

Crustacean

Aquatic arthropod like a crab, lobster, or shrimp.

Jointed appendage

Leg or limb with joints that allow flexibility and movement.

Malpighian tubule

Excretory organ that removes nitrogen waste in insects.

Metamorphosis

Process of transformation from larva to adult.

Molting (Ecdysis)

Shedding the exoskeleton to grow a new one.

Spiracle

Small external opening that allows air to enter the tracheal system.

Thorax

Middle body region where legs and wings attach.

Trachea (Tracheal tubes)

Internal air tubes used for respiration.

Endoskeleton

Internal skeleton made of calcium plates under the skin.

Echinoderm

Marine animal with radial symmetry and a water vascular system.

Madreporite

Filtered opening that draws water into the water vascular system.

Spine

Hard, pointed projection for protection.

Tube foot

Suction-cup structure used for movement, feeding, and respiration.

Water vascular system

Network of water-filled tubes that control movement and feeding.

Blastula

Early embryonic stage that is a hollow ball of cells.

Body cavity

Space inside an organism that holds internal organs.

Deuterostome

Organism whose anus develops before the mouth during development.

Embryo

Early stage of development after fertilization.

Evolutionary complexity

Level of body organization and system development in animals.

Gastrula

Embryonic stage where tissue layers begin to form.

Tissue

Group of similar cells performing a specific function.