THE WORLD OF LIFE with Douglas Drenkow

Although there is a true "coelom" (membrane-lined body cavity outside of the gut), which in many other animals provides shape to the body, by "hydrostatic (water) pressure", the body of an echinoderm is typically shaped by a spiny, stony "endoskeleton" -- mesh-like and flexible, like a Medieval knight's coat of mail, and only thinly covered by softer tissues.

The body plan of most echinoderms is more or less "radially symmetrical" (circular in design) and "pentamerous" (the body parts developing in groups of five); however, that radial symmetry is "secondary" -- the presumed ancestors of echinoderms, like the present-day larvae, are "bilaterally symmetrical" (the body divisible into only two mirror halves, not a larger number of mirror images, as in the radial design of the adults).

Typically, a great many tiny "tube feet" stick-out from the body wall.

Sea-lilies and feather-stars have five, often branched, feathery arms on their radially symmetrical body, with the mouth and (off-center) anus both pointing upwards (unlike the other, presumably more advanced members of this phylum). The tube feet are located by a groove in each branch of the arms. Growing up to two-feet long, sea-lilies are attached to the bottom by a flexible, backbone-like stalk; and feather-stars (more advanced) separate from their stalk when mature and swim about, pausing occasionally to perch on rocks and coral by means of some claw-like structures on the underside of their body.

Starfish typically have five arms broadly joined to the central disk of the radially symmetrical body, typically one-half to one foot wide; in some species, the arms are so broadly joined to the central disk that they are apparently absent -- the starfish is simply a pentagon (a five-sided figure). The usually colorful body of a starfish is smooth to rough or obviously spiny. The coelom (membrane-lined body cavity outside of the gut) fills most of the central disk and most of the space within the arms. The tube feet are present within a groove running on the underside of each arm from its tip in to the animal's mouth, pointing downward from the center of the underside of the central disk; the anus points upward, from the center of the upperside of the central disk.

Brittle stars, basket stars, and the like typically have five, often extensively branched, spiny, brittle arms narrowly joined to the central disk of the radially symmetrical body, which is usually not very colorful. The central disk of a brittle star may be about half an inch across; that of a basket star, about half a foot across. The coelom fills little of the arms, which are instead filled mostly with stony, segmented pieces that look like the "vertebrae" of vertebrates. The tube feet are found not in a groove but simply on the underside of the arms. The mouth is on the underside of the body.

Sea urchins, heart urchins, and sand dollars have no arms on their typically radially symmetrical body, which measures up to a few inches across and is encased within a thinly covered, spiny, stony shell. Sea urchins typically crawl over (but sometimes burrow into) rocky bottoms. The somewhat flattened, ball-shaped body of a sea urchin has five rows of tube feet running from the area of the mouth, on the underside of the body, "longitudinally" (vertically) up around to the area of the anus, on the upperside of the body. Heart urchins, ball-shaped, and sand dollars, disk-shaped and often slotted, typically burrow into sandy bottoms. They typically have a somewhat bilaterally symmetrical body (the mouth and/or anus are somewhat off-center) and a thinly covered, hard shell, covered with tiny spines.

Sea cucumbers have no starfish-like arms; instead, their body is cucumber- or worm-shaped, leathery, radially (somewhat bilaterally) symmetrical, measuring from a few inches up to a foot long, with only microscopic stony spines (although warty growths, looking like those on a cucumber, are often present). Typically bushy tentacles (actually modified tube feet) often grow around the mouth, at the opposite end of the body from the anus. Crawling sea-cucumbers typically have rows of tube feet, especially well developed on the underside of the body (which is, thus, somewhat bilaterally symmetrical); whereas wormlike, burrowing sea-cucumbers typically have small or no tube feet.

FEEDING HABITS

Most echinoderms are carnivorous or scavenging; some are herbivorous.

MOTION

Well-developed muscles flex the arms of echinoderms and operate their tube feet.

As muscles squeeze a tiny bulb inside the body, increased water pressure extends the attached tube foot outward and slightly forward; it then attaches to the seafloor, by means of an adhesive pad; as other muscles subsequently contract, the body of the animal is pulled forward.

The fluid pressure in the many tube feet is maintained by one-way valves connected to the echinoderm's "water-vascular system" -- a system of bodily canals that develops from the "coelom" (the membrane-lined body cavity outside of the gut) -- a system possibly descended from the coelom-filled tentacles of a "lopophore" (a feeding apparatus like that employed by modern-day moss animals).

In such echinoderms as sea urchins and starfish, this water-vascular system opens on the upper body surface, through a "sieve plate", which filters water taken-in from the environment; this intake is located on the lower surface of brittle stars.

Feather stars swim, by means of the muscular, yet graceful movement of their arms. Brittle stars creep, by means of the muscular movement of their highly flexible arms. Sea urchins crawl, by means of the hydraulic action of their tube feet accompanied by the coordinated muscular action of their spines; and heart urchins and sand dollars dig themselves into the sand, by means of the action of the many tiny spines on the outside of their body.

In addition, the rough surface of many echinoderms is kept clean of parasites by the brushing action of hair-like cilia and by the muscular actions of highly specialized, sometimes poisoned, microscopic jaws!

DIGESTION

The gut of echinoderms is typically "complete", with both a mouth and an anus.

In sea-lilies and feather stars (and the presumed ancestors of echinoderms, similar to lopophorates), tube feet on the arms, waved through the water, catch food particles and move them into the central groove in each arm, where the particles are trapped by mucous and carried by cilia in to the mouth.

Starfish typically cover their prey with their entire body: See the note, below, about the "eversible" stomach.

Brittle stars may use the five stony "jaws" around their mouth to chew the food they scavenge, they may sweep matter off the seafloor and into their mouth with their arms, or they may collect food particles with a sweep through the water of their sometimes mucous-webbed arms and then use their tube feet to carry the trapped material into their mouth.

Sea urchins consume the meals they scavenge with a large, specialized, five-toothed jaw, "Aristotle's lantern".

In sand dollars, food particles that pass in between the fine spines on the body are collected and carried by cilia and tube feet into the mouth.

Crawling sea-cucumbers use their tentacles to catch or collect food particles and bring them to their mouth; whereas burrowing sea-cucumbers (like earthworms) consume sand and digest its organic matter.

Between the mouth and the anus of a starfish, there is a large stomach, which in some species is "eversible" (able to be pushed-out over the prey) and in some other species is slipped in between the tight-fitting valves of their bivalve prey (such as clams); above this large stomach, there is a small stomach, which receives tube-like ducts from a pair of fern-leaf-like, food-absorbing "digestive glands" running within the length of each arm; and above this second stomach, there is a short "intestine" (with various outpocketings).

The gut of a sea urchin consists of an enlarged, five-toothed jaw (Aristotle's lantern); a tubular "esophagus"; a coiled, flattened stomach and intestine; and an anus. In addition, there is a by-pass tube, from the mouth cavity directly to the intestine.

The gut of sea-cucumbers consists of a large Aristotle's lantern; a stomach; and a long, coiled intestine, ending in an enlarged rectum -- which, because it also receives gametes (sperms or eggs) from the gonads, is termed a "cloaca".

RESPIRATION

Dissolved gases and other materials diffuse by osmosis (through selective cell membranes) across the body wall, especially through any infoldings or outgrowths -- such as the tube feet of many species, the tiny fingerlike outgrowths covering starfish, the five feathery gills on the underside of sea urchins, or the five petal-like gill-areas on the upperside of sand dollars.

Inside of sea-cucumbers, there are branching "respiratory trees" connected to the cloaca (rectal chamber), which pumps water (with dissolved gases) in and out. When distressed, some sea-cucumbers shoot sticky threads out of their cloaca at would-be attackers; and some can even "eviscerate" their (replaceable) gut and respiratory trees out through their cloaca or mouth.

CIRCULATION

The coelom and its accompanying "water-vascular system" (as described above) carry dissolved gases and other materials. In sea urchins, the coelomic fluid is circulated through the external gills.

EXCRETION

Dissolved materials diffuse by osmosis (through selective cell membranes) across the body wall, especially through any infoldings or outgrowths -- such as the tube feet of many species, the tiny fingerlike outgrowths covering starfish, the five feathery gills on the underside of sea urchins, the five petal-like gill-areas on the upperside of sand dollars, or the internally branching respiratory trees connected to the pumping rectal cloaca of sea-cucumbers.

COORDINATION

The growth, development, and activities of echinoderms are under genetic and hormonal control, as influenced by the environment.

With no central brain, the nerves of an echinoderm are typically arranged radially (in keeping with the basically circular body design). The nervous system of an echinoderm is typically very efficient, especially in coordinating the complex movements of the typically enormous number of tube feet. With no "cephalization" (with no distinct and highly sensory head), most of these radially symmetrical animals can move in any direction with equal ease; however, heart urchins and sand dollars -- somewhat bilaterally symmetrical -- typically move in only one direction.

REPRODUCTION

Some echinoderms can reproduce "asexually" (without mates). For example, select portions of the central disk of a starfish can, if detached, re-generate a whole new animal, in a process similar to the intact disk re-generating a lost arm. Likewise, the brittle arms of a brittle star allow the creature to escape with just the temporary loss of a limb if attacked.

Echinoderms typically reproduce sexually, typically with separate (male and female) sexes; however, some brittle stars are "hermaphroditic" (each individual with both egg- and sperm-producing gonads). Gametes (eggs and sperms) are typically released into the watery environment; however, many cold-water echinoderms brood their eggs on their body. The larvae of echinoderms are bilaterally symmetrical (as were the presumed ancestors of this group). In the extremely complicated development of a bilaterally symmetrical larva into a radially symmetrical adult, the mouth forms on one side of the body: The "face" of an echinoderm (such as the disk of a starfish) is technically a "lateral" surface.

Animals (Metazoa)

Doug@DouglasDrenkow.com

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

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