Annelids and Allied Taxa
Dividing up the Body
• The fluid-filled coelom is an efficient
hydrostatic skeleton
• When septa divided this coelom into a series
of compartments, metamerism first arose
• Evolutionarily, if one segment should fail,
another could still function
• Metamerism makes independent segmental
movements possible
Characteristics
• The three phyla are Annelida (marine and
freshwater worms, leeches), Echiura, and
Spincula
• All three phyla have a trochophore larva
• We will spent most of our time understanding
the phylum Annelida
Annelids
• About 15,000 species of segmented worms
• 2/3 of them are obscure, marine species not
noticed very easily
• The nervous system is more centralized and the
circulatory system is fairly complicated
• Except for leeches (Class Hirudinea), annelids
have tiny chitinous bristles called setae
• Short setae anchor a segment in an earthworm
(for example) to prevent it from slipping
backward
• Long setae help aquatic worms swim
The ecology of annelids
• They occur in the sea, freshwater, and on land
• Marine annelids borrow in the mud or live in
tubes
• Some feed on organic matter in the mud,
some are suspension feeders, many are
predators
• Freshwater annelids burrow in the mud, live
among vegetation, or swim freely
• Leeches are bloodsuckers or are carnivorous
Economic importance
• Many annelids are grazers and consumers of
detritus in food chains; many are preyed upon by
fish
• Burrowing on land or in oceanic mud and sand,
annelids are important for drainage, aeration,
mixing of soil and distribution of organic matter
– A study by Charles Darwin showed that the lowly
earthworm brings as much as 8 -20 tons of soil to the
surface each year
• Blood-sucking leeches are used medicinally
Anatomical Features
• Two part head has anterior tip containing the
prostomium and peristomium
• Terminal portion bearing the anus is the
pygidium
• The prostomium and pygidium are not true
segments
• New metameres form just in front of the
pygidium; thus the newest segments are at
the posterior
Anatomical Features
• Peritoneum (mesodermal epithelium) lines the body wall and
forms dorsal and ventral mesenteries
• Septa are between adjacent segments
• Except in leeches, the coelom is filled with fluid and serves as
a hydrostatic skeleton
– The fluid volume remains constant
– Therefore contraction of longitudinal muscles causes the
body to shorten and fatten
How annelids move
– Contraction of circular muscles causes the body to narrow
and lengthen
– By separating this force into sections, widening and
elongation move the whole animal
– Alternate waves of contraction or peristalsis result in
efficient burrowing
– Swimming annelids use undulatory movements
Anatomy of the Annelids
More Anatomy
Class Polychaeta (the bristle worms)
• The largest class of annelids with more than 10,000
species, mostly marine
• From 1 mm to 10 feet long
• Some live in crevices or in tubes; some swim and are
pelagic
• They are an important component of the food web
• Polychaetes have a well differentiated head with
sensory organs
• Paired appendages called parapodia are on most
segments
Characteristics of the polychaetes
• They have no clitellum as in earthworms
• Many setae are arranged in bundles on the parapodia
• The head or prostomium may or may not be
retractable; it often bears eyes, tentacles and sensory
palps
• The first segment or peristomium surrounds the mouth
and may have setae, palps, or chitinous jaws
• Suspension feeders may bear a tentacular crown that
opens like a fan but can be withdrawn into a tube
Segments of the trunk
• Most segments bear parapodia with lobes, cirri,
setae and other parts
• Parapodia have two lobes, the dorsal notopodium
and the ventral neuropodium
• Usually the parapodia are the chief respiratory organ
although the worm may also possess gills
• The parapodia help the worm to crawl, swim or serve
as an anchor in a tube
• Along with gills, they serve as chief respiratory
organs
• Amphritite has plumelike gills
Amphitrite
• Builds its tubes in mud or sand
• Extends long tentacles over the mud to gather
bits of organic matter as food
Tube-dwelling sedentary polychaetes
• The Christmas tree worm, Spirobranchus giganteus, lives is a
calcareous tube
• The sabellid polychaete, Bispira brunnea, lives in a leathery
tube
Sense organs of polychaetes
• Sense organs include eyes and statocystes that
are more developed than in the class
Oligochaeta
• The eyes vary from simple eyespots to well
developed eyes that can distinguish images
Reproductive characteristics
• Polychaetes have no permanent sex organs
and sexes are separate
• The gonads appear as temporary swellings of
the peritoneum
• Gametes are shed into the coelom and exit by
gonoducts, metanephridia or rupturing of the
body
• Fertilization is external and the early larva is a
trochophore
Diversity of the Polychaetes
• These worms are pelagic, burrowers, sometimes
sedentary some live in tubes
• Nereis, the clam worm, is a predatory polychaete
with jaws which can deliver a noticeable bite
when handled
• Featherduster worms or fan worms live in tubes
but unfurl tentacular crowns to feed
• Fanworms are polychaete ciliary feeders,
directing small food balls along grooved radioles
to the mouth by ciliary action
Sabella, a polychaete suspension feeder
Chaetopterus
• A sedentary polychaete which has wing-like notopodia that
secrete mucous filters to strain out food particles from the
water
• As the net fills with food, the food cup rolls it into a ball which
is passed into a groove toward the mouth
Arenicola
• This polychaete lives in an L-shaped burow in
intertidal mudflats and ingests food-laden
sand
Reproductive behavior of polychaets
• Most of the time, polychaetes live as sexually
immature worms called atokes
• When swollen with gametes, they are called
epitokes
• At the beginning of the last quarter of the
October-November moon, the polychaetes enter
a “swarming period”
• The epitokes break off and swim to the surface;
swarming ensures that all epitokes mature at the
same time
Class Oligocheta
• Over 3000 species occur in habitats from soil
to freshwater; a few are marine or parasitic
• Nearly all have setae but in general, less setae
than polychaetes
• Earthworms burrow in moist, rich soil,
emerging at night
• In wet weather, they stay near the surface.
When it is dry, they burrow deep and become
inactive
More on earthworms
• Lumbricus terrestris is often studied in biology
classes
• Charles Darwin studied earthworms and
estimated that 10 to 18 tons of dry earth passed
through earthworm intestines per acre annually
• The worms help churn the soil, mixing materials,
and adding nutrients
• Giant tropical earthworms may reach 4 meters
long and live in interconnected tunnels
Form and function of oligochaetes
• The prostomium overhangs the mouth at the
anterior end
• In most earthworms, each segment bears four
pairs of chitinous setae, some may bear over
100
• Each seta is a bristle-like rod set in a sac and
moved by tiny muscles
• The setae anchor segments during burrowing
How earthworms move
• Earthworms move by peristalsis
– Circular muscles contract, the anterior end lengthens, setae anchor
the forward end
– Longitudinal muscles contract, body shortens, and the posterior end is
pulled forward
Feeding habits of Oligochaetes
• Most are scavengers, feeding on decayed organic
matter, leaves, refuse, etc
• Food is moistened by the mouth and drawn in by
a sucking action of the muscular pharynx
• Digestion and absorption occur in the intestine;
an infolded typhlosole increases the surface area
• Chloragogen tissue surrounds the intestine and
helps in digestion
• The chloragogen cells also function in excretion
Circulation, respiration, and excretion
• Both coelomic fluid and blood carry food,
wastes and respiratory gases
• Blood circulates in a closed system with five
main trunks running lengthwise in the body
• The dorsal blood vessel above the alimentary
canal has valves and functions as a true heart
• The dorsal vessel pumps blood anteriorly into
five pairs of aortic arches
• Blood contains colorless amoeboid cells and
dissolved hemoglobin
Circulation, respiration, and excretion
• Gaseous exchange (oxygen and carbon dioxide)
occurs through the moist skin
• Each somite (segment) except the first three and the
last one have a pair of nephridia
• Each nephridium occupies parts of two adjacent
segments
• A ciliated funnel, the nephrostome, draws in wastes
and leads through the septum
• These coil until the nephridial duct ends at a bladder
that empties outside at the nephridiopore
• Wastes from the coelom are discharged
Nervous system and sensory organs
• A pair of cerebral ganglia connects around the
pharynx to the ganglia of the ventral nerve
cord
• Fused ganglia in each somite contain both
sensory and motor fibers
• One or more giant axons are located in the
ventral nerve cord to increase the rate of
conduction and stimulate contractions of
muscles in many segments
Reproduction and Development