LECTURE NOTES ON ZOOLOGY

Module 4. MOLLUSCS, CHAETOGNATHS, PHORONIDS, BRYOZOANS, BRACHIOPODS, POGONOPHORANS, HEMICHORDATES, ECHINODERMS

Content Module 4.2. Hemichordates, echinoderms

Topic 4.2.1. Phylum Hemichordata

General characteristics of the phylum. Origin, evolution, paleontological Background, modern diversity, geographic distribution, and their role in nature and human life. A review of major taxonomic groups important for human life and the functioning of natural ecosystems. Class Enteropneusta. Class Pterobranchia.

Exclusively marine benthic animals, intolerant of low salinity. Most species inhabit warm seas. Both solitary burrowing forms and attached sessile colonial organisms occur. Approximately 100 extant and over 1,000 extinct species. Bilaterally symmetrical deuterostome coelomate animals with the body divided into three regions: the proboscis, collar, and trunk. Each region contains its own coelomic sacs: an unpaired sac in the proboscis and paired sacs in the collar and trunk. The cavities of the proboscis and collar communicate with the external environment via coelomoducts, whereas the trunk coelomic sacs are closed. A characteristic feature is the stomochord (notochord)—a small, blind, elastic outgrowth of the entodermal Pharynx projecting forward into the proboscis and serving as its support—as well as paired metameric gill slits connecting the gut to the exterior. The digestive tract is complete; the Circulatory system is well-developed with orderly Blood flow driven by the pulsation of a closed pericardial sac. The Nervous system forms a continuous nerve plexus with condensations of Nerve Cells into dorsal and ventral cords. Gonochoristic; Gonads are located in the trunk region. Fertilization is external.

The phylum Hemichordata includes two classes: Enteropneusta and Pterobranchia.

Class Enteropneusta.

Free-living, benthic, worm-like animals with a predominantly burrowing lifestyle, ranging in length from a few centimeters to 2.5 m. About 70 species. The body is distinctly divided into three regions. The proboscis is acorn- or oval-shaped; it is a muscular Structure well adapted for burrowing. The Base of the proboscis narrows into a slender neck embraced by the second, also muscular body region—the collar—which rises above the body surface as a prominent ridge. The trunk accounts for 9/10 of the body length. The anterior part of the trunk is perforated on the sides by two rows of numerous narrow gill slits. The coloration is generally yellowish-brown or brown, covered with a thick layer of mucus produced by numerous mucous cells, which are particularly abundant on the proboscis and collar. The body is covered by a delicate ciliated epithelium containing A large number of unicellular glands and sensory nerve cells. The epithelium rests on a thin, structureless basal membrane, beneath which lie two Muscle layers: an outer circular and an inner longitudinal layer. Each body region features well-developed specialized Muscles (pharyngeal muscles, and muscles that bend and straighten the proboscis). On the inner side, the longitudinal cutaneous muscles are lined by the ciliated peritoneal epithelium of the coelomic sacs. The coelomic cavity of the proboscis communicates with the exterior via a short canal with a pore on the DORSAL SIDE OF the proboscis (rarely two canals). Similar canals with pores are present in each of the collar coelomic sacs; they open into the first pair of gill slits. Excess fluid is eliminated through the proboscis pore, while the collar pores serve to fill its coeloms with Water, regulating the volume and turgor of the proboscis necessary for burrowing. The trunk coeloms do not communicate with the exterior. In the collar and trunk, the coelomic sacs join above and below the gut to form dorsal and ventral mesenteries; however, both mesenteries in the collar and the dorsal mesentery in the trunk are often incomplete, allowing the cavities of the right and left sacs to communicate with each other.

Class Pterobranchia.

A relict group of marine animals, comprising about 30 extant and nearly 1,000 extinct species. Extant species are grouped into three genera: Atubaria, Cephalodiscus, and Rhabdopleura; almost all are colonial, sessile animals. Individual zooids are small; the largest species, Cephalodiscus densus, reaches 1.4 cm, but most species are smaller, ranging from 0.2 to a few millimeters. Colony sizes span several centimeters (up to 25 cm in height and about 19 cm in width). The body is divided into three regions. The proboscis, or cephalic shield, is flattened and contains an unpaired coelomic cavity connected to the exterior by two pores; its ventral side bears glands that produce a Chitin-like substance used to build the tubes in which the animals live. The collar possesses a paired coelom, from which one or several (up to 8) arm-like outgrowths containing coelomic cavities extend. Each arm bears two rows of hollow, ciliated tentacles. The shortened trunk terminates in a stalk or peduncle of variable length. The Muscular System has lost the characteristics of a cutaneous-muscular sac due to the sessile mode of life. The collar contains strong arm muscles, the contraction of which retracts the arms into the tube (their extrusion outward occurs through an increase in pressure within the collar coeloms while the pores are closed and the wall muscles contract), as well as pharyngeal muscles and two bundles of powerful muscles extending to the cephalic shield and fanning out to attach to its ventral wall. Additionally, there are longitudinal muscles in the trunk wall and stalk muscles that extend along the ventral side of the trunk to the collar. Their contraction draws the animal into the tube.