9.2 Marine Worms and Some Small Invertebrate Phyla

Bilateral symmetry

The arrangement of body parts into two identical halves, featuring distinct anterior and posterior ends and dorsal and ventral surfaces.

Anterior

The defined front end of a bilaterally symmetric animal

Posterior

The defined rear end of a bilaterally symmetric animal

Dorsal

The upper or back surface of an animal with bilateral symmetry

Ventral

The underside or belly surface of an animal with bilateral symmetry

Mesoderm

The middle of embryonic tissue that gives rise to muscles, the reproductive system, and other organs

Proboscis

An extension near the mouth that in some invertebrates helps capture or collect food

Hydrostatic skeleton

A system that uses water pressure against the body shape and aid in locomotion

Segmentation

A body consisting of similar compartments or segments

Coelom

The body cavity found in structurally complex animals

Parapodium

A usually flat lateral extension present on each of the body segments of polychaetes

Setae

The bristles of polychaetes

Morphology

body shape

Closed circulatory system

A circulatory system where the blood or circulatory fluid is contained in distinct vessels

Gill

Thin-walled extensions of the body that are used in gas exchange

Capillaries

Thin blood vessels that allow for gas exchange

Trochophore

A planktonic larva found in polychaetas, some mollusks, and other invertebrates

Swarming

The aggregation of individuals for spawning or other purposes