L34 (Animal Diversity I: Overview and Early Invertebrates) Key Terms

cleavage

succession of rapid cell divisions without significant growth during early embryonic development that converts the zygote to a ball of cells

blastula

a hollow ball of cells that marks the end of the cleavage stage during early embryonic development in animals

gastrulation

in animal development, a series of cell and tissue movements in which the blastula-stage embryo folds inward, producing a three-layered embryo, the gastrula

gastrula

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

larva

a free-living, sexually immature form in some animal life cycles that may differ from the adult animal in morphology, nutrition, and habitat

metamorphosis

a developmental transformation that turns an animal larva into either an adult or an adult-like stage that is not yet sexually mature

body plan

in multicellular eukaryotes, a set of morphological and developmental traits that are integrated into a functional whole, the living organism

radial symmetry

the body parts are arranged around a single main axis that passes through the center of the animal

bilateral symmetry

the body parts are arranged around two axes of orientation, the head-tail axis, and the dorsal-ventral axis

endoderm

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 in species that have these structures.

ectoderm

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.

mesoderm

The middle primary germ layer in a triploblastic animal embryo; develops into the notochord, the lining of the coelom, muscles, skeleton, gonads, kidneys, and most of the circulatory system in species that have these structures

diploblastic

having two germ layers

triploblastic

possessing three germ layers: the endoderm, mesoderm, and ectoderm. All bilaterian animals are triploblastic

body cavity

a fluid- or air-filled space between the digestive tract and the body wall

coelom

a body cavity lined by tissue derived only from mesoderm

protostomes

in animals, a developmental mode distinguished by the development of the mouth from the blastopore; often also characterized by spiral cleavage and by the body cavity forming when solid masses of mesoderm split

deuterstomes

in animals, a developmental mode distinguished by the development of the anus from the blastopore; often also characterized by radial cleavage and by the body cavity forming as outpockets of mesodermal tissue

blastopore

in a gastrula, the opening of the archenteron that typically develops into the anus in deuterostomes and the mouth in protostomes

invertebrates

animal without a backbone

filter feeders

an animal that feeds by using a filtration mechanism to strain small organisms or food particles from its surroundings

choanocytes

a flagellated feeding cell found in sponges. Also called a collar cell, it has a collar-like ring that traps food particles around the base of its flagellum

amoebocytes

an amoeba-like cell that moves by pseudopodia and is found in most animals. Depending on the species, it may digest and distribute food, dispose of wastes, form skeletal fibers, fight infections, or change into other cell types

hermaphrodites

an individual that functions as both male and female in sexual reproduction by producing both sperm and eggs

polyp/medusa

the sessile variant of the cnidarian body plan/the floating, mouth-down form of the cnidarian body plan

cnidocyte

a specialized cell unique to the phylum Cnidaria; contains a capsule-like organelle housing a coiled thread that, when discharged, explodes outward and functions in prey capture or defense

nematocysts

in a cnidocyte of a cnidarian, a capsule-like organelle containing a coiled thread that when discharged can penetrate the body wall of the prey

gastrovascular cavity

a central cavity with a single opening in the body of certain animals, including cnidarians and flatworms, that functions in both the digestion and distribution of nutrients

medusozoans

all cnidarians that produce a medusa are members of clade Medusozoa

anthozoans

these cnidarians occur only as polyps

exoskeleton

a hard encasement on the surface of an animal, such as the shell of a mollusc or the cuticle of an arthropod, that provides protection and points of attachment for muscle