Cva

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Eye and pineal gland cross section

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Analogous

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Two structures that perform the same or similar function by a similar mechanism but evolved separately.

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Anatomy

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The branch of morphology that deals with the structure of animals

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Clade

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A group of biological taxa or species that share features inherited from a common ancestor

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Cladogram

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A branching dendrogram representing the organization and relationships of clades

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Dendrogram

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A branching diagram that represents the relationships of the history of a group of organisms.

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Derived condition

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Denoting an organism of species that evolved late within its phylogenetic lineage; advanced; opposite of primitive.

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Extant

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means 'living' organisms as opposed to extinct organisms

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Extinct

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no longer in existence

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Heteroocercal

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a tail with the upper lobe larger than the lower and with the vertebral column prolonged into the upper lobe

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Homocercal

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a symmetrical tail that extends beyond the end of the vertebral column (as in most bony fishes)

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Homologous

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having the same evolutionary origin but not necessarily the same function; "the wing of a bat and the arm of a man are homologous"

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Homoplasy

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Features in two or more organisms that look alike; similar in appearance

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Index fossil

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A fossil animal widely distributed geographically but restricted to one rock layer or time horizon; defining species indicator of a stratum.

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Macroevolution

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evolution on a large scale extending over geologic era and resulting in the formation of new taxonomic groups

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Metamerism

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the property of having repeated segments

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Microevolution

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the occurrence of small-scale changes in allele frequencies in a population, over a few generations, also known as "change below the species level."

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Monophyletic

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a taxon (group of organisms) which forms a clade, meaning that it consists of an ancestor and all its descendants.

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Ontogeny

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changes in an organism from zygote to death, although often focused on events from zygote to maturity.

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Paraphyletic

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a taxon consisting of a single ancestor but in which one or some of its descendants are omitted

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Phylogeny

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the course of evolutionary change within a related group of organisms.

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Polyphyletic

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a taxon which has descendants from more than one ancestral taxon, or put another way, a group that consists of members that do not share a common ancestor

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Preadaptation

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a situation where an organism uses a preexisting anatomical structure inherited from an ancestor for a potentially unrelated purpose. One example of preadaptation is dinosaurs having used feathers for insulation and display before using them to fly, or sweat glands in mammals being transformed into mammary glands.

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Primitive condition

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a descriptive term often used in the field of evolution to describe particular species or traits that are characteristic of an older evolutionary scale of development relative to more recent developments.

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Taxon

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A particular group of organisms of any taxonomic rank. For example, a phylum, a genus, or a species.

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Ascidian tadpole

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free-swimming larva of ascidians; they have a tail like a tadpole that contains the notochord - Urochordata

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Cephalochordata

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chordate subphylum defined by the presence of a notochord that persists throughout life. It is represented in the modern oceans by the lancelets (amphioxus).

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Cleavage

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a rapid series of cell divisions that follows fertilization and produces a multicellular blastula.

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Coelom

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the fluid filled body cavity within the mesoderm.

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Deuterostomes

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an animal whose anus forms from or near the embryonic blastopore; the mouth forms at the opposite end of the embryo.

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Ectoderm

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the start of a tissue that covers the body surfaces. It emerges first and forms from the outermost of the germ layers.

Generally speaking, the ectoderm differentiates to form the nervous system, tooth enamel and the epidermis (the outer part of integument)

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Endoderm

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the inner germ layer that develops into the lining of the digestive and respiratory systems

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Endostyle

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a longitudinal ciliated groove on the ventral wall of the pharynx which produces mucus to gather food particles. It is found in urochordates and cephalochordates, and in the larvae of lampreys (homologous to the thyroid gland in vertebrates)

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Hemichordata

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is a phylum of worm-shaped marine deuterostome animals, generally considered the sister group of the echinoderms. They seem to have a primitive form of notochord, formed from a diverticulum of the foregut called a stomochord, but this is most likely the result of convergent evolution rather than homology with the vertebrate notochord.

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Larva

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an immature (nonreproductive) stage that is morphologically different from the adult.

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Mesoderm

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the middle germ layer that develops into muscle and bone and cartilage and blood and connective tissue

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Metamorphosis

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the marked and rapid transformation of a larva into an adult that occurs in some animals

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Monoecious

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having male and female reproductive organs in the same plant or animal

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Notochord

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a flexible rodlike structure that forms the supporting axis of the body in the lowest chordates and lowest vertebrates and in embryos of higher vertebrates

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Pharyngeal slit

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The wall of the pharynx is perforated by up to 200 vertical slits, which are separated by stiffening rods.

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Protochordate

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any member of either of two invertebrate subphyla of the phylum Chordata: the Tunicata (sea squirts, salps, etc.) and the Cephalochordata (amphioxus).

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Actinopterygii

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A taxonomic class within the superclass Osteichthyes — the ray-finned fish

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Amniote

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Birds, reptiles and mammals are all amniotes; that is, their embryos are enclosed within an extraembryonic membrane, the amnion, which contains amniotic fluid. This provides a 'private pond' for the developing embryos of these land-dwelling vertebrates.

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Amphibians

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such as frogs, toads, salamanders, newts, and caecilians, are ectothermic (or cold-blooded) animals that either metamorphose from a juvenile water-breathing form, to an adult air-breathing form, or paedomorph and retain some juvenile characteristics.

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Chondrichthyes

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cartilaginous fishes are jawed fish with paired fins, paired nares, scales, two-chambered hearts, and skeletons made of cartilage rather than bone.

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Conodont element

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are extinct chordates resembling eels, classified in the class Conodonta. For many years, they were known only from tooth-like microfossils now called conodont elements, found in isolation.

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Cotylosauria

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A member of the Captorhinidae, the earliest group of true reptiles from the Carboniferous through the Permian.

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Cyclostomes

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Any of various primitive jawless fish of the class Cyclostomata, such as the lamprey or hagfish

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Dipnoi

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bony fishes of the southern hemisphere that breathe by a modified air bladder as well as gills; sometimes classified as an order of Crossopterygii

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Elasmobranchii

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Elasmobranchs are cartilaginous fish that have an upper jaw that is not fused to the braincase, no swim bladder, an advanced electroreceptive system, a spiracle, skin with placoid scales, teeth that are modified placoid scales, and 5-7 separate slit-like gill openings. These include the 600 species of sharks, skates, and rays.

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Eutherians

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Mammals that give birth to live offspring (viviparous) and possess an allantoic placenta.

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Gnathostome

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a vertebrate animal possessing true jaws

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Gymnophiona

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an order of amphibians including caecilians

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Holocephali

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a taxonomic subclass, within class Chondrichthyes - the ghost sharks or chimaera

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Intervertebral disk

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a fibrocartilaginous disc serving as a cushion between all of the vertebrae of the spinal column (except between the first two)

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Lepidosauria

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a taxonomic superorder, within infraclass Lepidosauromorpha - the scaled reptiles

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Lissamphibia

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The subclass Lissamphibia includes all recent amphibians and means smooth amphibia.

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Mammalia

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are a class of vertebrate animals whose females are characterized by the possession of mammary glands while both males and females are characterized by sweat glands, hair, three middle ear bones used in hearing, and a neocortex region in the brain.

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Metatherians

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Any of the pouched animals of the infraclass Metatheria; a marsupial; Belonging or pertaining to the infraclass Metatheria of marsupials

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Neural spine

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A dorsal, posteriorly directed spine on top of the neural arch (more information).

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Osteichthyes

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also called bony fish, are a taxonomic group of fish that includes the ray-finned fish (Actinopterygii) and lobe-finned fish (Sarcopterygii).

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Ostracoderms

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are any of several groups of extinct, primitive, jawless fishes that were covered in an armor of bony plates.

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Parareptilia

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is a subclass or clade of reptiles which are variously defined as an extinct group of primitive anapsids, or a more cladistically correct alternative to Anapsida.

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Placodermi

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extinct group of bony-plated fishes with primitive jaws

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Quadruped

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an animal especially a mammal having four limbs specialized for walking

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Reptilia

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class of cold-blooded air-breathing vertebrates with completely ossified skeleton and a body usually covered with scales or horny plates; once the dominant land animals

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Salientia

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frog: any of various tailless stout-bodied amphibians with long hind limbs for leaping; semiaquatic and terrestrial species

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Synapsida

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A clade containing organisms having a single opening in the skull behind each eye, the temporal fenestra. The sole surviving synapsids are the mammals, but there were many odd and highly successful non-mammalian synapsids in the past. Mammal-like reptiles

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Teleostomi

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a clade of jawed vertebrates that includes the tetrapods, bony fish, and the wholly extinct acanthodian fish. Key characters of this group include an operculum and a single pair of respiratory openings, features which were lost or modified in some later representatives. The teleostomes include all jawed vertebrates except the chondrichthyans and the placodermi.

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Tetrapod

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a vertebrate animal having four feet or legs or leglike appendages

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Therapsida

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extinct mammal-like reptiles found inhabiting all continents from the mid Permian to late Triassic

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Urodela (Caudata)

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a taxonomic order, within subclass Lissamphibia - the salamanders and related amphibians

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Vertebrae

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is an individual bone in the flexible column that defines vertebrate animals, e.g. humans. The vertebral column encases and protects the spinal cord, which runs from the base of the cranium down the dorsal side of the animal until reaching the pelvis.

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Allantois

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an extraembryonic extension of the hindgut of amniote embryos that functions in excretion and sometimes in repiration.

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Amnion

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a saclike membrane that holds the developing embryo in a compartment of water

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Blastocoel cavity

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the fluid-filled cavity inside a blastula

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Chorioallantoic membrane

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the very vascular fetal membrane composed of the fused chorion and adjacent wall of the allantois

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Chorion

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the outermost membranous sac enclosing the embryo in higher vertebrates (reptiles, birds and mammals)

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Cleavage

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a rapid series of cell divisions that follows fertilization and produces a multicellular blastula.

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Coelom

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the fluid-filled body cavity formed within the mesoderm.

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Dermatome

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an embryonic skin segment

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Ectoderm

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the outer germ layer that develops into skin and nervous tissue

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Endoderm

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the inner germ layer that develops into the lining of the digestive and respiratory systems

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Epimere

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The epimeric mesoderm is the most dorsal region. It separates longitudinally into discrete clumps of mesoderm termed somites. Each somite is further split into dermotome, myotome, and sclerotome segments. Epimeric mesoderm is ultimately fated to further differentiate into dermis, body muscles and vertebrae.

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Extraembryonic

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pertaining to a structure formed by or around the embryo but not retained by or directly contributing to the adult body.

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Gastrula

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early embryonic stage during which the basic gut if formed.

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Gastrulation

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the process in which a gastrula develops from a blastula by the inward migration of cells

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Heterochrony

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within an evolutionary lineage the change in time at which a characteristic appears in the embryo relative to its appearance in a phylogenetic ancestor; usually concerned with the time of onset of sexual maturity relative to somatic development. Compare with paedomorphosis.

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Holoblastic

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early mitotic planes pass entirely through the cleaving embryo. Compare with discoidal cleavage and meroblastic.

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Hox genes

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a group of related genes that specify the anterior-posterior axis and segment identity of metazoan organisms during early embryonic development. These genes are critical for the proper number and placement of embryonic segment structures (such as legs, antennae, and eyes).

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Hypomere

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The hypomeric mesoderm is the most ventral region. It is fated to further differentiate into limbs, peritoneum, gonads, heart, blood vessels and mesenteries.

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Induction

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refers to the initiation or cause of a change or process, such as the production of a specific morphogenetic effect in the developing embryo.

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Isolecithal

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pertaining to an egg in which the yolk is evenly distributed throughout the cytoplasm.

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Macrolecithal

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pertaining to eggs with large quantities of stored yolk.

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Meroblastic

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early mitotic planes that do not complete their passage through the embryo before subsequent division planes form.

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Mesenchyme

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loosely associated cells of mesodermal origin. mesodermal tissue that forms connective tissue and blood and smooth muscles

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Mesoderm

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the middle germ layer that develops into muscle and bone and cartilage and blood and connective tissue

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Mesolecithal

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pertaining to eggs with moderate amounts of stored yolk.

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Mesomere

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a midzone of the mesoderm between the epimere and hypomere.

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Microlecithal

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having little or no yolk. Microlecithal development is the pattern of embryonic development characteristic of eggs with little or no yolk.

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Morula

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a solid mass of blastomeres that forms when the zygote splits; develops into the blastula

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Myotome

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undifferentiated embryonic blocks of presumptive muscles.

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Neural crest

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a paired strip of tissue that separates from the dorsal edges of the neural groove as it forms the neural tube.

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Neurula stage

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The stage of a vertebrate embryo when gastrulation is largely finished and a neural plate is forming, ending with formation of the neural tube.

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Neurulation

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the process of forming a dorsal ectodermal tube -- the neural tube.

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Ontogeny

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changes in an organism from zygote to death, although often focused on events from zygote to maturity.

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Oviposition

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the act of laying egss by oviparous animals.

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Paedomorphosis/ Neoteny

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The retention of general juvenile features of ancestors in the late developmental stages of descendants.

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Parturition

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the act of giving birth via vivparity.

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Peramorphosis

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a phylogenetic change in which individuals of a species mature past adulthood and take on hitherto unseen traits. It is the reverse of pedomorphosis. Peramorphosis can also be defined as the delaying of maturity while the development of the adult is extended.

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Placenta

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a composite organ formed of maternal and fetal tissues through which the embryo is nourished.

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Sclerotome

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A sclerotome is part of a somite, a structure in vertebrate embryonic development. Sclerotomes eventually differentiate into the vertebrae and most of the skull.

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Senescence

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the organic process of growing older and showing the effects of increasing age

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Somatic mesoderm

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the outer wall of the hypomere. In association with ectoderm, it is fated to form limbs, gonads, and the peritoneum.

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Splanchnic mesoderm

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the inner wall of the hypomere. In association with endoderm, it differentiates into the heart and blood vessels, mesenteries, and (in mammals), extra-embryonic membranes.

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Somatopleure

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the somatopleure is a structure created during embryogenesis when the lateral mesoderm splits into two layers. The outer (or somatic) layer becomes applied to the inner surface of the ectoderm, and with it forms the somatopleure.

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Somite

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Mesodermal structures formed during embryonic development that give rise to segmented body parts such as the muscles of the body wall.

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Splanchnopleure

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the splanchnopleure is a structure created during embryogenesis when the lateral mesoderm splits into two layers. The inner (or splanchnic) layer adheres to the endoderm, and with it forms the splanchnopleure.

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Telolecithal

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also known as macrolecithal, refers to the uneven distribution of yolk in the cytoplasm of ovums of found in birds, reptiles, and fish. The yolk is concentrated at one pole of the egg separate from the developing embryo.

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Zygote

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fertilized ovum ((genetics) the diploid cell resulting from the union of a haploid spermatozoon and ovum (including the organism that develops from that cell))