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The Diversity of

The World of Life

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The Diversity of The World of Life

Animals (Metazoa)

Segmented Worms

(Annelida)

Representatives

Mostly Marine Worms, such as Bristleworms & Ragworms (Polychaeta)

Earthworms & Leeches (Oligochaeta, or Clitellata)

Biology

APPROXIMATE NUMBER OF KNOWN SPECIES WORLDWIDE

12,000

ENVIRONMENTS

Polychaetes swim in the sea, crawl over the seafloor, or burrow into it (straight down or in a U-shaped pattern, with two openings to the surface).  Earthworms burrow through moist soil, and closely related species live in freshwater (and sometimes in tubes they make).  Leeches live in saltwater or freshwater, in moist Tropical areas, or (as parasites) on the bodies of higher animals.

DESCRIPTION

"Hydrostatic" support (water pressure) -- from fluid within the typically segmented "coelom" (membrane-lined body cavity outside the gut) -- gives shape to the segmented ("metameric") body.  Typically, "setae" (bristles) stick-out from the body.

The typically cylindrical, minute to quite lengthy body of polychaetes is equipped with many setae, typically borne like claws on "parapodia" (paired, segmented, paddle-like to finger-like lobes on either side of the body segments).  The head of many polychaetes bears small fingerlike growths (antennae and/or "palps") or large tentacles or feathery growths; and many polychaetes live within tubes they form, often of grains of sand or bits of shell cemented together with a secretion.

The body of earthworms is cylindrical, highly segmented, and equipped with fewer setae than on typical polychaetes.  Although closely related, similarly structured aquatic species are typically less than an inch long and most earthworms are just a few inches long, some earthworms grow up to 10 feet long!

The body of a leech is typically one to two inches long (up to a foot long in some species), fairly "dorso-ventrally" flattened (flattened from top-to-bottom), and ringed (However, there are more external rings than actual, internal segments to the body, as defined by the internal organs).  Leeches have no setae.  There is a sucker (a modified body segment) around the mouth and another at the tail-end.

FEEDING HABITS

Polychaetes are scavenging, filter-feeding, herbivorous, or carnivorous; earthworms are scavenging (working air into the soil as they burrow, consuming organic debris, and leaving their rich "castings" (manure) behind); and leeches are either predatory on invertebrates or parasitic on invertebrates or vertebrates.

MOTION

Annelid worms have typically well-developed "longitudinal" muscles (running lengthwise through several segments) and "circular" muscles (within each body segment).  This segmentation makes possible "peristaltic waves" of muscular contractions, prehistorically an adaptation to burrowing (as seen in modern earthworms):  As the setae on one group of body segments are pushed-out into the soil, the setae on the segments ahead of them are pulled inward, out of the soil; and the circular muscles in the leading segments tighten, like a belt, squeezing these segments forward.  Then, as the setae on the leading segments are pushed-out into the soil, the setae on the following segments pulled inward, out of the soil; and the longitudinal muscles in the following segments shorten, pulling these segments forward.

In polychaetes, waves of muscular contractions on one side of the body and then on the other are used to coordinate the walking or paddling action of the parapodia.

Leeches are especially muscular -- many crawl by "looping" like inchworms, attaching and releasing their head-end and tail-end suckers, one after the other; and some leeches swim, with waves of muscular contractions running down the entire length of their body.

DIGESTION

The gut of annelid worms is "complete", with both a mouth and an anus.

Filter-feeding polychaetes typically have ciliated, mucous-covered tentacles or feathery growths from the head, which trap food particles and carry them to the mouth.

Predatory polychaetes have jaws on a muscular "pharynx", which they can extend out of their mouth to catch prey.

The pharynx of an earthworm pumps-in soil, mixed with food particles.

Many leeches feed with an extendable, tubular "proboscis"; and many blood-sucking leeches feed through their mouth-sucker, with the aid of surgically sharp jaws, a "anaesthetic/anti-coagulant" secretion (to deaden the pain and keep the blood flowing), and a blood-sucking pharynx.

The gut of an annelid is continuous through all the body segments.

In an earthworm, behind the pharynx there is an "esophagus", often modified into a "crop", which stores food, and one or more muscular "gizzards", which help grind food.  Behind the esophagus there is the "intestine", bearing an inner fold, which aids in the absorption of nutritious materials, mixed-in with the soil the worm consumes.  A layer of cells around the intestine of an earthworm functions like the liver of a vertebrate, in "intermediary metabolism" (biochemically transforming food molecules).

In a leech, between the pharynx and the intestine, the gut bears an esophagus and a large "crop" -- a stomach, with many pouches, able to store blood between passing "hosts".

RESPIRATION

Gases diffuse by osmosis, through selective cell membranes, across the body wall of annelids.

Simple osmosis is sufficient for terrestrial species, whose body wall is kept moist, as by secretions of mucous, and is filled with vessels containing oxygen-absorbing blood.

Polychaetes in burrows or tubes create a "ventilating" current of water (carrying dissolved oxygen gas) that passes over their body by means of such bodily motions as "peristaltic waves" (See above).  In many polychaetes, the usually leg-like paurapodia are modified as gills; and in some species, gills may form as outgrowths near the head or from a modified filter-feeding organ.

EXCRETION

Although some species have salt- and water-balancing systems like the blind-ended "protonephridia" of flatworms, most annelids have open-ended "metanephridia", more like those in mollusks -- a pair of tubes opens into and collects dissolved wastes from the coelom (body cavity) of each body segment and carries them (through the "septum", dividing the segments) into the body segment behind it, where the tubes loop around and are in close contact with blood vessels, which re-absorb any valuable materials; the resulting urine is excreted out through a pore.  There is more water and less salt in the urine of freshwater (and terrestrial) oligochaetes than in the urine of saltwater polychaetes.

CIRCULATION

Dissolved gases and other materials are carried throughout the body of annelids by a typically "closed" circulatory system.  Blood is pumped anteriorly (forwards) by and through a "dorsal artery" (a large blood vessel within the upperside of the body).  Vessels within segments in the head-end of the worm (modified into five pairs of so-called "hearts" in common earthworms) carry the blood around either side of the gut down to a large "ventral" vessel (in the underside of the body), through which the blood flows posteriorly (rearward) (Compare lancelets and vertebrates).  Branches from the ventral vessel carry the blood up into the tissues within each of the body segments and eventually back up to the dorsal artery.  The coelom (the membrane-lined body cavity outside of the gut) also carries some dissolved gases and other materials -- some leeches, in particular, have lost their blood vessels and have evolved more or less a "hemocoel" (a blood-filled body cavity, similar to that in mollusks).

COORDINATION

The growth, development, and activities of annelid worms are under genetic and hormonal control.

Although most annelids are poorly "cephalized", free-swimming, crawling, or otherwise active polychaetes do have a well-developed head.  In most annelids, nervous-system inputs and outputs are processed by a (so-called) "brain" -- lying within the dorsal (upper) portion of the head-end -- and a "double nerve cord" -- consisting of two nerve cords running side-by-side down the length of body, within the ventral (lower) portion of the segments.  Although it is continuous throughout the length of the body, the double ventral nerve cord is typically enlarged (as "ganglia") within each body segment, to control nerve functions in that particular segment.  The "integument" covering the body of earthworms is highly sensitive, as to light.

REPRODUCTION

Some annelids can reproduce asexually -- certain segments of certain segmented worms can regenerate whole new worms.

Most annelids are "hermaphroditic" (each individual has both male and female "gonads", producing sperms or eggs), although polychaetes have separate sexes.  In polychaetes, gametes are produced by the "peritoneum" (the membranous lining of the coelom body cavity) in a few to many of the body segments; and the gametes exit the body typically through the excretory, kidney-like metanephridia.  Polychaetes fertilize their eggs in the open sea, although in many species, the males and females of the species swarm together in great groups.  Most terrestrial annelids "copulate" (mate with physical contact), with a transfer of sperm from each hermaphroditic partner to the other.  In leeches, the sperm is bundled into "spermatophores", which some leeches actually inject into one another.  In earthworms and leeches, the gametes are produced by gonads in specialized segments; and certain body segments are modified as a typically collar-like "clitellum", which secretes a cocoon, in which the eggs are fertilized and the young will develop.  The young of earthworms and leeches look like small adults -- development is "direct" (without a dissimilar, larval stage).  The young of polychaetes is a "trochophore larva" (like that of the probably related mollusks) -- bearing a tuft of cilia at the "head" end, a ring of cilia around the middle, and the mouth and anus in the lower half.  During the development of an annelid, new segments are added just ahead of the tail segment.

Earthworms, which typically surface only at night or during rains, typically burrow deeply into the soil and often lie dormant during periods of cold or dry weather.

Animals (Metazoa)

Doug@DouglasDrenkow.com

(c) 2004 D.D.  All Rights Reserved.

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