Invertebrate Zoology: A Study Guide - T. A. Dauda 2014

Metazoa
Echinoderms

General characteristics

Echinoderms are coelomate animals that exhibit radial body Symmetry in their adult stage. In most species, Organs are arranged along 5 radii, though some have a different number of rays. While the radial symmetry of Cnidarians is primary, in echinoderms it is secondary, as their ancestors possessed bilateral body symmetry. The free-swimming larvae of echinoderms are bilaterally symmetrical. Echinoderms are characterized by the presence of a Water-vascular (ambulacral) system, which serves for locomotion and participates in Respiration and excretion. The secondary body cavity (coelom) is well developed and filled with coelomic fluid. Echinoderms are exclusively marine animals. They are predominantly benthic organisms capable of slow movement across the substrate, or more rarely, attached to it. Some echinoderm species are of commercial importance.

As first demonstrated by the research of I.I. Mechnikov, echinoderms are of significant interest for elucidating the phylogenetic relationships between invertebrates and Representatives of the phylum Chordata. Despite the radial symmetry of adult individuals, the Organization and development of echinoderms share many common features with Chordates. Similar to chordates, their coelom is formed by the Separation of mesodermal pouches from the archenteron. Like chordates, they are deuterostomes, in which the embryonic blastopore either closes or develops into the anus, while the larval Mouth forms anew. Representatives of both phyla possess a two-layered integument and skeletal elements of mesodermal origin. These similarities suggest that lower chordates are phylogenetically linked to echinoderms through common ancestors. Fossil remains of echinoderms have been found in deposits dating back to the Paleozoic era.

The integument of echinoderms consists of 2 layers: an outer layer resembling a single-layered epithelium, and an inner layer formed by Fibrous Connective Tissue. Various elements of the calcareous dermal Skeleton develop within the inner layer. In sea stars, these take the form of calcareous plates arranged in longitudinal rows (along the rays), which typically bear outward-projecting spines. In sea urchins, the body is enclosed in a rigid calcareous test composed of tightly joined rows of plates bearing long movable spines. In sea cucumbers (holothurians), small calcareous ossicles of various shapes are scattered throughout the dermis.

Musculature is developed to varying degrees depending on mobility and The Nature of the dermal skeleton. It is composed of individual Muscles and muscular bands.

The water-vascular (ambulacral) system begins with a porous madreporite located on the aboral (dorsal) surface of the body. A stone canal extends from it deep into the body, opening into a ring canal that surrounds the Esophagus. The ring canal gives rise to radial canals extending into each ray of the body. Short lateral canals branch off from the radial canals in both directions; from these, internal contractile vesicles (ampullae) project into the body cavity, while external contractile tubular tube feet with terminal suckers project outward. The water-Vascular System is filled with water entering through the madreporite. When the ampullae contract, water is forced from them into the lumen of the tube feet, causing them to elongate and stretch out. The terminal suckers adhere to the substrate, after which the tube feet shorten as the water is pumped back into the ampulla. Through the combined efforts of many simultaneously contracting tube feet, the body of the echinoderm is pulled forward, and the animal slowly moves along the sea floor. Thanks to the suckers of the tube feet, echinoderms are able to crawl.

The Nervous system of echinoderms has a radial Structure. Radial nerve cords, corresponding in number to the body rays, extend from the circumoral nerve ring.

Sense Organs are poorly developed. Primitive ocelli (eyes) are located at the tips of the rays in sea stars, and on the upper surface of the body in sea urchins. Organs of Touch and other sensory structures are also present.

Digestive System. In most species, the mouth is located centrally on the lower (oral) surface of the body. The mouth leads into a short esophagus, followed by the midgut and a short hindgut. An anus is absent in some species.

The respiratory organs of sea stars and sea urchins are dermal branchiae (Skin gills)—thin-walled outgrowths on the upper surface of the body. The water-vascular system apparently also plays a role in respiration. In A number of echinoderms, gas exchange occurs directly across the body surface.

The Circulatory system typically consists of 2 ring vessels—one surrounding the mouth and the other the anus—and radial vessels, the number of which corresponds to the number of body rays. Both ring vessels are connected by a hematopoietic axial organ permeated by a network of Blood Vessels.

Excretory organs. Echinoderms lack specialized excretory organs. The Elimination of Metabolic waste products occurs through the walls of the water-vascular system canals and via specialized amoeboid Blood Cells (coelomocytes) that migrate within the Organism.

Reproductive organs vary in structure. Most echinoderms are dioecious, but hermaphroditic forms also exist. Development involves a series of complex metamorphoses. The bilaterally symmetrical larvae of echinoderms swim freely in the water Column. Many echinoderms possess a high capacity for the regeneration of body parts. For example, a single severed ray of a sea star can regenerate an entire organism.

The phylum Echinodermata is divided into 5 classes: Asteroidea (sea stars), Ophiuroidea (brittle stars), Crinoidea (sea lilies and feather stars), Echinoidea (sea urchins), and Holothuroidea (sea cucumbers). The most widespread classes are Asteroidea, Echinoidea, and Holothuroidea.