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)
|