(embryology) the repeated division of a fertilised ovum


The hollow ball of cells marking the end stage of cleavage during early embryonic development


An embryonic stage in animal development encompassing the formation of three layers: ectoderm, mesoderm, and endoderm.


the immature free-living form of most invertebrates and amphibians and fish which at hatching from the egg is fundamentally unlike its parent and must metamorphose


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


A group of organisms that share the same level of organizational complexity or share a key adaptation.


phylum porifera (sponges)


member of the clade of 'true animals', the animals with true tissues (all animals except sponges)


Members of the radially symmetrical animal phyla, including cnidarians.

radial symmetry

the property of symmetry about an axis


Members of the branch of eumetazoans possessing bilateral symmetry.

bilateral symmetry

arrangement of body parts so there are distinct left and right halves that mirror each other





germ layers

Tissue layers in the embryos of all animals except sponges, ectoderm, mesoderm, endoderm


The outermost of the three primary germ layers in animal embryos; gives rise to the outer covering and, in some phyla, the nervous system, inner ear, and lens of the eye


The innermost of the three primary germ layers in animal embryos; lines the archenteron and gives rise to the liver, pancreas, lungs, and the lining of the digestive tract


The endoderm-lined cavity, formed during gastrulation, that develops into the digestive tract of an animal.


Having two germ layers.


has three germ layers: the ectoderm, endoderm, and mesoderm.


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


an animal that lacks a coelom, or body cavity

body cavity

A fluid-containing space between the digestive tract and the body wall.


an organism in which the ceolom is not completely covered by the mesoderm


An animal that possesses a true coelom (a body cavity lined by tissue completely derived from mesoderm).


a body cavity completely lined with mesoderm


Organisms that first forms a mouth when it develops from the blatopore.


Undergoes radial cleavage and the anus forms before the mouth.

spiral cleavage

A type of embryonic development in protostomes, in which the planes of cell division that transform the zygote into a ball of cells occur obliquely to the polar axis, resulting in cells of each tier sitting in the grooves between cells of adjacent tiers.

determinate cleavage

A type of embryonic development in protostomes that rigidly casts the developmental fate of each embryonic cell very early.

radial cleavage

A type of embryonic development in deuterostomes in that the planes of cell division that transform the zygote into a ball of cells are either parallel or perpendicular to the polar axis, thereby aligning tiers of cells one above the other.

indeterminate cleavage

A type of embryonic development in deuterostomes, in which each cell produced by early cleavage divisions retains the capacity to develop into a complete embryo.


In protosomes, a pattern of development in which a solid mass of mesoderm cells splits to form the cavity that becomes the coelom.


in deuterostomes when development begins as buds from the archenteron


The opening of the archenteron in the gastrula that develops into the mouth in protostomes and the anus in deuterostomes


a clade that encompasses the annelids, mollusks, and several other phyla, they are distinguished by two morphological features, the lophophore, a crown of tentacles used for feeding,and the trochophore larva, a distinct larval stage


One of two distinct clades within the protostomes. It includes the arthropods and animals that molt as they grow.

trochophore larva

Distinctive larval stage observed in some lophotrochozoan animals, including some annelids and molluscs.

Ediacaran period

the last period during the Precambrian period, when there was extensive glaciation, which delayed further evolution on earth

Cambran explosion

545 to 525 mya all major body plans evolved examples are in the fossils of the Burgess Shale


water is circulated and food removed through a funnel fringed with cilia

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