Posterior portion of the body of arthropods consisting of similarly formed segments, containing the reproductive organs and a part of the alimentary canal. In insects and arachnids, it is the posterior section of the body.


A structural, physiological, or behavioral trait that allows an organism to live in its environment.

Ambulacral Groove

In echinoderms, any of the radial grooves through which the hydraulic system's tube feet protrude.


An organism that can live without free oxygen.


Animals with a long cylindrical body consisting of ring-formed segments.


A pair of long sensory appendages on the head of many arthropods.


An eight-legged arthropod.


A scientist who studies arachnids-spiders and related groups.


An animal with articulated appendages and a segmented body, covered by an exoskeleton.

Asexual Reproduction

Any reproductive process, such as the production of gemmae or the division of a cell or organism into two or more approximately equal parts, that does not involve gametes joining together.

Bilateral Symmetry

Corporal form whereby the right and left halves of an organism are approximate mirror images of each other.


The science that studies living organisms-their constitution, structure, function, and relations.


A group of marine invertebrates whose soft body is protected by a shell consisting of two parts called valves.


A form of the chemical compound calcium carbonate.

Carrion Eaters

An animal that feeds on dead animals it finds. Given the occasion, some large carnivores such as lions and hyenas can behave like carrion eaters.


A social group that carries out specific tasks, characteristic of ants and bees, among other insects.


A cavity formed between layers of mesoderm in which the alimentary tract and other internal organs are suspended.


A class of exclusively marine mollusks with tentacles or legs attached to the head. These appendages have rows of suckers that are used for capturing prey and copulation.


The head and thorax combined in one single body segment.


First pair of appendages in crabs, sea spiders, and arachnids, usually in the form of pincers or fangs.


Tough, durable polysaccharide that contains nitrogen and is found in the exoskeleton of arthropods or other surface structures of many invertebrates, and also in the cell walls of fungi.


One of the many divisions into which scientists classify animals. The invertebrates form a separate class of their own.


The process of establishing, defining, and ordering taxa within a hierarchical series of groups.


A protective sheath usually made of silk. Many insects make cocoons to protect themselves during the pupa stage, until they become adults.


A group of animals of the same species that live and work together to survive.


The entire population of organisms that inhabit an environment in common and who interact with one another.

Compound Eye

In arthropods, a complex eye made of many separate units, each of which has light-sensitive cells and a lens that can form an image.


An animal of the arthropod group, with antennae and articulated appendages, that uses gills to breathe and has a body protected by a thick covering.


An organic, noncellular protective covering secreted by the epidermis.


The part of an organism's digestive process that consists of eliminating undigested matter.


The internal layer of the skin below the epidermis.


An animal in which the anus is formed in or near the developing embryo's blastophore zone, and whose mouth is formed afterward in another location; the echinoderms and the chordates are deuterostoma.


One species that exists in two distinct forms.


Invertebrate marine animals. The bodies of the adults have a pentagonal symmetry. Underneath the skin they have a calcareous skeleton with spines and protuberances. They have an internal hydraulic system, connected with ambulacral feet, that makes locomotion possible.


Native to a particular geographical region and restricted to it.


One of the three layers of the embryonic tissue of animals; it originates in the epithelium that covers certain internal structures.


The thin, outermost layer of the arthropod exoskeleton, consisting primarily of wax.


The outermost layer of cells.

Epithelial Tissue

Type of tissue that surrounds a body or structure or covers a cavity. Epithelial cells form one or more regular layers with little intercellular material.


The comparative study of animal behavior in its natural habitat, and the evolutionary, genetic, ecological, and physiological factors that influence its manifestation.


The changes in the genetic reservoir from one generation to the next, as a consequence of processes such as mutation or natural selection, among other things.


The external covering supporting the body, commonly found in arthropods. It is like an articulated shell made of chitin; it serves as a support for muscles and the soft internal organs.


Simple light receptor, common among invertebrates.


A category in taxonomy that groups genus together; lower than order and higher than genus.


The preserved remains of an organism that disappeared a long time ago.


The mature reproductive cell that combines with a gamete of the opposite sex to form a zygote that is usually diploid; male gametes are called spermatozoids and female gametes are called ovules.

Gastrovascular Cavity

A digestive cavity with an opening, characteristic of the phyla Cnidaria and Ctenophora. It has digestive and circulatory functions.


A category in taxonomy that groups species together.


A directional response to gravity.


A pore in the reproductive apparatus through which gametes pass.


A blood-filled cavity inside the tissues; characteristic of animals with an incomplete circulatory system, such as mollusks and arthropods.


An organism that has both reproductive systems, male and female; hermaphrodites may or may not self-fertilize.


An organic molecule, secreted in small amounts by one part of an organism, that regulates the function of other tissue or organs.


An organism in which a parasite lives.

Hydrostatic Skeleton

A skeleton in which fluid is contained by muscular walls that transfer the force from one part of the body to another when subjected to pressure.


Relating to a species or organism that was brought into an environment and harms biodiversity, agricultural or fishing productivity, or human health.


Animal without a spinal column. Some, such as worms, have soft bodies. Others, such as arthropods, are protected by a hard exoskeleton.


Taxonomic category that includes phyla or divisions. Until the appearance of the category of domain, the kingdom was the highest-level category in biological classification.


Animal in a developmental stage, after leaving the egg. It can feed itself but has not yet acquired the shape and structure of the adults of its species.


Appendage immediately below the antennae, used to trap, hold, bite, or chew food.


In mollusks, the outer layer of the body wall or a soft extension of it. It usually secretes a shell.


Element or substrate where organisms live.


The middle layer of the three layers of embryonic tissue.


Abrupt transition from the larval form to the adult form.


Main group in the animal kingdom (including mollusks, annelids, and arthropods) in which the mouth is formed at or near the blastula in the developing embryo.


Organism that can be seen only with a microscope.


Seasonal travel of animals from one region to another to reproduce or to seek food, better climate, or better living conditions in general.


Property of certain animals and plants to resemble living things or inanimate objects that live nearby, mostly by means of color.


Invertebrates of the phylum Mollusca, with a soft body divided into a head, foot, and visceral mass. They have a fold called a mantle that envelops all or part of the body.


Removal of all or part of the outer covering of an organism; in arthropods, a periodic changing of the exoskeleton that enables them to grow in size.


Chemical elements essential for life. Examples are carbon, oxygen, nitrogen, sulfur, phosphorus, magnesium, and potassium.


The simple visual unit of a compound eye in arthropods; it contains light-sensitive cells and a lens that can form an image.


Living being that feeds on plants and animals.


Taxonomic category that includes families; category lower than a class and higher than a family.


Body part made of various tissues grouped into a structural and functional unit.

Organic Material

Animal or plant material in any stage of decomposition, found on or within the soil.


Any living creature, whether single-celled or multicellular.


Organism that lives at the expense of another.


Taxonomic category that includes classes; category lower than a kingdom and higher than a class.


Group of small living beings, whether plants (phytoplankton) or animals (zooplankton), that live suspended in freshwater or ocean water.


Type of unattached, ciliated larva of many organisms of the phylum Cnidaria (jellyfish, sea anemones, and coral).


The immobile stage in the life cycle of animals of the phylum Cnidaria.


Organism that feeds on other living beings.


Body cavity consisting of a fluid-filled space between the endoderm and the mesoderm, characteristic of nematode worms.


Temporary cytoplasmic projection of an amoeboid cell whose movement and feeding occur through phagocytosis.

Radial Symmetry

The regular disposition of body parts around a central axis in such a way that any plane that cuts through the axis divides the organism in halves that constitute mirror images of each other. It is seen in adult echinoderms.

Safety Thread

A silk thread that a spider leaves behind when it is moving, attaching it from time to time to various surfaces.



Group of individuals of the same species that live in a certain area during a specific time.

Measurement of the amount of common salt in water or soil. Common salt is a sodium salt, sodium chloride, common in nature, that gives a salty flavor to ocean water and salt lakes.


Successive cell divisions in the egg of an animal to form a multicellular blastula.

Sexual Dimorphism

An assembly of external morphological characteristics that make it possible to distinguish the males from the females of the same species.

Sexual Reproduction

Reproduction involving meiosis and fertilization.

Social Insects

Insects that live with others of the same species, looking after the young and gathering food for the community.


A group of individuals that recognize one another as belonging to the same reproductive unit.


One of the external openings of the respiratory system in terrestrial arthropods.


A balance organ consisting of a sac-like structure that contains grains of sand (statoliths) or some other material that stimulates the sensory cells when the organism is in motion.


The surface that constitutes an organism's habitat or life support.


Insects that act in a group for eating, mating, or finding a new location for a nest.



The process of segment formation (metameres) into corporal regions (tagmata) with differentiated functions.


Also known as taxia, it is the orientation of movement in those organisms that, being able to move freely from one place to another, track their course in the direction of an external stimulus.

Study of the principles of scientific classification. The organization, grouping, and denomination of living things.


Long and flexible organs located around the mouth of many invertebrates, often prehensile and tactile.


In crustaceans and insects, the fused segments located between the head and the abdomen to which the legs are attached.


Group of similar cells organized in a structural and functional unit.


In insects and some other terrestrial arthropods, the system of air conduits covered with chitin.


Chemical agent injected into other animals in order to kill or paralyze them, or to ward off an attack.


Discipline or science dedicated to the study of animals.

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