| A | B |
|---|
| acoelomate | lacking a cavity between the gut and outer body wall |
| anterior | head end of a bilaterally symmetrical animal |
| archenteron | develops into the digestive tract of an animal |
| bilateral symmetry | entral longitudinal plane that divides the body into two equal but opposite halves |
| bilaterian | animals with bilateral symmetry |
| blastopore | opening of the archenteron in the gastrula that form mouth in protostomes and anus in deuterostomes |
| blastula | hollow ball of cells marking the end stage of cleavage |
| body cavity | fluid-containing space between the digestive tract and the body wall |
| body plan | morphological and developmental traits that define a grade (level of organizational complexity) |
| Cambrian explosion | burst of evolutionary origins when most of the major body plans of animals appeared |
| cephalization | concentration of sensory equipment on the anterior end of the body |
| cleavage | succession of rapid cell divisions without growth during early embryonic development |
| coelom | body cavity completely lined with mesoderm |
| coelomate | possesses a true coelom |
| determinate cleavage | rigidly casts the developmental fate of each embryonic cell |
| deuterostome development | developmental mode distinguished by the development of the mouth from the blastpore |
| diploblastic | having two germ layers |
| dorsal | pertaining to the back of a bilaterally symmetrical animal |
| ecdysozoan | protostome development that some systematists hypothesize form a clade |
| ectoderm | gives rise to the outer covering |
| Ediacaran fauna | earliest generally accepted animal fossils |
| endoderm | gives rise to the liver |
| enterocoelous | mesoderm buds from the wall of the archenteron and hollows |
| eumetazoan | animals with true tissues (all animals except sponges) |
| gastrula | three-layered |
| gastrulation | formation of a gastrula from a blastula |
| germ layers | three main layers that form the various tissues and organs of an animal body |
| grade | group of animal species that share the same level of organizational complexity |
| indeterminate cleavage | early cleavage divisions retains the capacity to develop into a complete embryo |
| larva | free-living |
| lophophore | circular fold of the body wall bearing ciliated tentacles that surround the mouth |
| lophotrochozoan | characterized by lophophores or trochophore larvae |
| mesoderm | develops into the notochord |
| metamorphosis | development in an animal larva that transforms it into a sexually mature adult |
| parazoan | lacking true tissues |
| posterior | rear |
| protostome development | development of the anus from the blastopore |
| pseudocoelomate | body cavity is not completely lined by mesoderm |
| radial cleavage | aligning tiers of cells one above the other |
| radial symmetry | many equal parts radiating outward like the spokes of a wheel |
| schizocoelous | solid masses of mesoderm split |
| spiral cleavage | resulting in cells of each tier sitting in the grooves between cells of adjacent tiers |
| triploblastic | possessing three germ layers |
| trochophore larva | pistinctive larval stage observed in annelids and mollusc |
| ventral | underside |
