THE WORLD OF LIFE with Douglas Drenkow

Most cnidarians are marine species; some are freshwater. Some are "benthic" (living on the bottom), typically "sessile" (attached to the bottom) but occasionally burrowing; some are "planktonic" (drifting); and some are "pelagic" (swimming).

Masses of "stony coral" skeletons -- having grown one atop the other over the millennia -- form the well-known coral "reefs", which serve as uniquely sheltering environments for entire communities of organisms.

DESCRIPTION

Somewhat more complex than sponges but still simple animals, the cnidarians have actual "tissues" (specialized, coordinated assemblages of cells) but no true "organs" (specialized, coordinated assemblages of tissues), with the arguable exception of the gut (See below). There is no "coelom" (membrane-lined body cavity outside of the gut, as in higher animals) -- there is just a membranous to jellylike "mesoglea" in between the "epidermis" (the tissue covering the body) and the "gastrodermis" (the tissue lining the gut).

Cnidarians range in size from the microscopic (especially hydras) to the macroscopic (such as most jellyfish).

Although an "exoskeleton" (an encasing structure) composed of such materials as horny "chitin" or stony "calcium carbonate" supports and protects many colonial forms (as of corals), "hydrostatic" support (essentially water-pressure) -- from fluid in the gut -- typically gives shape to the "radially symmetrical" (basically circular or cylindrical) body, forming either a "medusa" or a "polyp" (a given species often having both forms, during different stages of its life).

A medusa is the form of a jellyfish -- the body is shaped like a parachute or bell. Tentacles hang down from the outer rim, and "oral arms" sometimes hang down from the (central) mouth.

A polyp is typically tubular, with tentacles around the mouth, at the top, and an anchoring stalk, at the bottom. In many species, numerous polyps grow together as branching colonies, often with a division of labor -- some polyps (with tentacles) may be feeding, other polyps (with buds) may be reproductive, and still others (as with stings) may be defensive: The Portuguese man-of-war consists of a colony of such specialized individuals, including a flotation "bladder".

FEEDING HABITS

Cnidarians are carnivorous, the prey often stung, adhered, or entangled by "nematocysts" (bulblike structures that shoot-out harpoon-like threads), found especially on the tentacles.

MOTION

The body parts of cnidarians are pulled into motion by primitive "contractile muscle fibers" in "dermal" (skin) cells (not true muscle cells, as in higher animals).

The pulsing contractions of the "bell" of a jellyfish provide a form of jet propulsion through the water.

DIGESTION

The saclike gut of a coelenterate has a mouth but no anus -- wastes are expelled through the mouth, as the body contracts. Foodstuffs, pre-digested by enzymes secreted into the gut and engulfed by specialized cells, diffuse by "osmosis" (through selective cell membranes) in through the body wall.

The mouth of sea anemones and corals bears a "pharynx" (an in-turned portion of the "epidermis", the tissue covering the body), leading to the gut, which is divided into sections -- like the segments of an orange -- by "mesentery" walls.

The gut, branched throughout the "bell" in medusae (such as jellyfish), carries foodstuffs and other dissolved materials, which diffuse by osmosis into and between the cells of the body and throughout the typically jellylike mesoglea.

RESPIRATION

Dissolved gases diffuse by osmosis through the body wall and between the cells of the body and throughout the mesoglea.

The gut (See above) carries not only digestive materials but sometimes also oxygen. There is no developed respiratory "system" of organs, as in higher animals.

CIRCULATION

The gut (See above) carries not only foods but also other dissolved materials. There is no developed circulatory "system" of organs, as in higher animals -- materials diffuse by osmosis between the cells of the body and throughout the mesoglea.

EXCRETION

Dissolved materials diffuse by osmosis throughout the mesoglea and between the cells of the body and out through the body wall.

The gut (See above) carries not only foods but also other dissolved materials. There is no developed excretory "system" of organs, as in higher animals.

COORDINATION

The structures and functions of cnidarians are ultimately under genetic control, as modified by environmental factors.

The activities, growth, and development of these multicellular organisms are presumably also coordinated by hormones (in effect, chemical messengers), as suggested by the observation that a polyp will "regenerate" (re-grow) lost tentacles only at the mouth end.

Cnidarians in general have nerve "nets" (although not complete nervous "systems", of organs, as in higher animals). Jellyfish in particular have light- and balance-sensors, chemically or "mechanically" connected to their nerve nets.

Nerve cells (in cnidarians and higher animals) actively conduct electrical impulses, carried by electrically charged "ions" (of salts). Throughout the Animal Kingdom in general, genetic and hormonal control -- both strictly chemical in nature -- coordinate long-term growth and development; whereas nervous control -- electro-chemical in nature -- coordinates quick muscular action.

REPRODUCTION

A cnidarian can reproduce asexually, typically by means of buds from specialized, reproductive polyps, which produce either new polyps or medusae.

For sexual reproduction, cnidarians usually have medusae of separate sexes -- male and female -- each producing "gametes" (sex cells, each with half the normal number of chromosomes) in false "gonads" on the surface of the body or gut (These are not true gonads, as in higher animals, because there is no outer layer of "sterile", non-reproductive, cells). The gametes are typically released into the water in the environment. The "sperms" (male gametes) use their propeller-like "flagellum" to actively swim to the drifting "eggs" (female gametes). The two gametes unite, forming a "zygote" (a fertilized egg, with both sets of chromosomes), which develops into "planula larva": This young animal -- solid, elongate, "radially symmetrical" (basically cylindrical), multi-cellular, multi-flagellated, and free-swimming or -creeping -- eventually settles down and grows into a new polyp (or colony of polyps), which will eventually produce more, mobile medusae and, thus, complete the life cycle.

In hydras, sea anemones, and corals, the trend in evolution has been for the polyp stage to be dominant over any medusa stage; but in jellyfish, the trend is for the medusa stage to be dominant over the polyp stage (which is typically represented just by a larval stage, eventually producing medusae like a stack of pancakes).

Animals (Metazoa)

Doug@DouglasDrenkow.com

Photo of Cells: H.D.A. Lindquist, US EPA

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