INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 3 - H.I. Shcherbak - 1997

DEUTEROSTOMIA

PHYLUM HEMICHORDATA

This phylum comprises exclusively marine benthic animals that cannot tolerate even the slightest decrease in salinity. Most species inhabit warm seas. They include both solitary burrowing forms and attached, sessile organisms that form peculiar colonies. Around 100 extant and more than a thousand extinct species have been described.

Hemichordates are bilaterally symmetrical deuterostome coelomate animals whose body is divided into three regions: the proboscis, collar, and trunk. Each region contains its own coelomic sacs: an unpaired one in the proboscis, and paired ones in the collar and trunk. The cavities of the proboscis and collar communicate with the external environment via coelomoducts, whereas the coelomic sacs of the trunk are closed.

Characteristic Features of hemichordates include the presence of a so-called notochord—a small, blind, elastic outpocketing of the endodermal Pharynx projecting forward into the proboscis and serving as its support—as well as paired, metameric gill slits through which the gut connects with the external environment.

The gut is complete, and There is a well-developed Circulatory system with organized Blood flow, driven by the pulsations of a closed pericardial sac. The Nervous system is represented by a continuous nerve plexus with concentrations of Nerve Cells forming dorsal and ventral cords.

Hemichordates are gonochoric (dioecious), with Gonads developing in the trunk region. Fertilization is external.

The phylum Hemichordata includes two classes: Enteropneusta and Pterobranchia.