Annelids and Allied Taxa - Unicamdocenti.unicam.it/tmp/2596.pdf · Annelids and Allied Taxa . ... annelids, and in vertebrates, ... Excretion Excretory organs

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CHAPTER 17

Annelids and Allied Taxa

Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

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Characteristics

Diversity

Annelids exhibit segmentation or metamerism

Bodies composed of serially repeated units

Each unit contains components of most organ

systems

Evolution of metamerism allowed much greater

complexity in structure and function

Increased burrowing efficiency by permitting

independent movement of segments

Evolution of a more sophisticated nervous system

Provided a safety factor

If one segment failed, others could still function


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Cleavage is spiral mosaic

Mesoderm forms from derivatives of the 4d cell

Coelom forms by schizocoely

Share a trochophore as the ancestral larval form

Segmented worms living in marine, freshwater, and moist terrestrial habitats

Include marine bristle worms, leeches, and earthworms, pogonophorans, and vestimentiferan worms

Characteristics


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Members of phylum Echiura and Sipuncula are benthic marine animals with unsegmented bodies

Molecular sequence data place echiurans within phylum Annelida

Echiurans Sister taxon to Annelida

Sipunculans Sister taxon to a clade composed of Annelida and

Echiura

Characteristics


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Phylum Annelida

Characteristics About 15,000 species

2/3 are the more obscure marine worms.

Segmentation Body segments marked by circular grooves

called annuli

Metamerism Repetition of organs in segments called metameres or

somites

Septa separate segments

Found in arthropods, probably homologous with annelids, and in vertebrates, where it evolved independently


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Setae Tiny chitinous bristles called setae

Absent in leeches

Short setae anchor segments in earthworms Prevent it from slipping backward

Long setae help aquatic worms swim

Polychaetes Primarily marine and usually benthic

Oligochaetes and leeches Freshwaters, or terrestrial soils

Many leeches are predators

Phylum Annelida


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Body Plan Body Wall

Head is composed of

Prostomium

Perstonium

Terminal portion bearing the anus is the pygidium

Head and pygidium are not considered metameres

New metameres form in front of the pygidium

Surface is covered with an epidermis and a thin outer layer of non-chitinous cuticle

Strong circular and longitudinal muscles underlie the body wall

Phylum Annelida


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Coelom

Forms by schizocoely

Peritoneum (mesodermal epithelium) lines

body wall and forms dorsal and ventral

mesenteries

Peritonea of adjacent segments meet to

form the septa

Gut and longitudinal blood vessels extend

through the septa

Phylum Annelida


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Hydrostatic Skeleton Except in leeches, coelom is filled with fluid and

serves as a hydrostatic skeleton

Fluid volume remains constant

Contraction of longitudinal muscles Causes body to shorten and expand

Contraction of circular muscles Causes body to narrow and lengthen

By separating this force into sections, widening and elongation move the whole animal

Alternate waves of contraction, or peristalsis, allow efficient burrowing

Swimming annelids use undulatory movements

Phylum Annelida


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Phylogeny Traditionally, annelids are divided among

3 classes Class Polychaeta

Class Oligochaeta

Class Hirudinida

Polychaeta is a paraphyletic class

Oligochaeta and Hirudinida form a monophyletic group called Clitellata

Characterized by reproductive structure called a clitellum

Phylum Annelida


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Some now consider Clitellata to be an annelid class containing oligochaetes and leeches as orders

We retain the three classes and consider Clitellata a clade whose members are class Oligochaeta and class Hirudinida

Class Oligochaeta is a paraphyletic group because ancestors of leeches arose from within it

Phylum Annelida


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Class Polychaeta

Diversity Largest class of annelids

More than 10,000 species, mostly marine

Vary from 1 mm to 3 meters long

Differentiation of some somites

More specialization of sensory organs than in clitellates

Tolerate a wide range of salinity

Warmer regions have more freshwater polychaetes

Some live in crevices, others inhabit tubes, or are pelagic


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Important part of marine food chains

Well-differentiated head with sense organs

Paired appendages called parapodia on most segments

No clitellum

Many setae arranged in bundles on parapodia

Sedentary polychaetes mainly tube-living

Errant polychaetes may be free-moving, burrowing, or crawling

Clamworm Nereis is an example of a predatory polychaete with jaws on an eversible, muscular pharynx

Class Polychaeta


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Form and Function Prostomium may or may not be retractile

Often bears eyes, tentacles, and sensory palps

Surrounds mouth and may have setae, palps, or chitinous jaws

Ciliary feeders may bear a tentacular crown that opens like a fan but can be withdrawn into the tube

Most segments of trunk bear parapodia with lobes, cirri, setae, and other parts Aid crawling, swimming, and anchor worm in a

tube

Usually the chief respiratory organ although the worm may also possess gills

Amphritite and Arenicola

Class Polychaeta


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Nutrition Polychaetes have a foregut, midgut and hindgut

Foregut has a stomodeum, pharynx, and anterior esophagus lined with cuticle

Midgut derived from mesoderm secretes enzymes and absorbs nutrients

Short hindgut derived from ectoderm and leads to anus on the pygidium

Errant polychaetes are predators or scavengers

Sedentary polychaetes feed on suspended particles or particles in sediment

Class Polychaeta


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Circulation and Respiration Most have parapodia and gills for gaseous

exchange

Others use the body surface

Circulation varies

In Nereis a dorsal vessel carries blood forward and a ventral vessel carries blood posteriorly

Blood flows across between these major vessels in networks around the parapodia and intestine

In some, septa are incomplete and coelomic fluid serves circulatory function

Many polychaetes have respiratory pigments

Hemoglobin, chlorocruorin or hemerythrin

Class Polychaeta


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Excretion Excretory organs vary, from protonephridia to

metanephridia, and mixed forms

One pair per metamere

Inner end (nephrostome) opens into the coelomic cavity

Coelomic fluid enters the nephrostome

Selective resorption occurs along the nephridial duct

Class Polychaeta


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Nervous System and Sense Organs Dorsal cerebral ganglia connect to subpharyngeal

ganglia by a circumpharyngeal commissure

Double ventral nerve cord runs length of the worm with ganglia in each metamere

Sense organs include Eyes, nuchal organs and statocysts

Eyes vary from simple eyespots to well-developed image-resolving eyes similar to mollusc eyes

Alciopid eyes have accessory retinas specialized for the wavelengths that penetrate deep seas

Nuchal organs are ciliated sensory pits that are probably chemoreceptive

Some burrowing and tube-building polychaetes use statocysts to orient their body

Class Polychaeta


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Reproduction and Development

Polychaetes have no permanent sex organs

Monoecious

Gonads appear as simple temporary swellings of

peritoneum

Gametes are shed into coelom and exit by

gonoducts, metanephridia, or rupturing of the

body

Fertilization is external and the early larva is a

trochophore

Class Polychaeta


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Representative polychaetes Clam Worms: Nereis

Errant polychaetes

Live in mucus-lined burrows near low tide level

Come out of hiding places at night to search for food

Prostomium bears a pair of palps sensitive to touch and taste, a pair of short sensory tentacles, and two small dorsal eyes sensitive to light

Peristomium has a ventral mouth, a pair of jaws, and four pairs of sensory tentacles

Class Polychaeta


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Parapodia

Each has two lobes Dorsal notopodium

Ventral neuropodium

One or more chitinous spines (acicula) support each lobe

Abundant blood vessels assist respiration

Function in creeping and swimming

Oblique muscles in each somite

Undulatory movements of the body provide free-swimming and burrow-pumping actions

Feed on small animals, other worms, and larval forms

Food moved through alimentary canal by peristalsis

Class Polychaeta


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Scale worms Belong to the family Polynoidae

Flattened bodies are covered with broad scales

Some are large, all are carnivores and some are commensals in burrows of other organisms

Fireworms Have hollow, brittle setae that contain poisonous

secretions

Feed on cnidarians

Class Polychaeta


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Tubeworms Tube-dwellers

May line their burrows with mucus

Use cilia or mucus to obtain food

Fanworms Unfurl tentacular crowns to feed

Food moved from radioles to mouth by ciliary action

Parchment Worms Lives in a U-shaped tube

Modified segments pump water through tube

Class Polychaeta


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Clade Siboglinidae (Pogonophorans)

Formerly members of phylum Pogonophora (beardworms)

Discovered in 1900

Characteristics 150 species described

Most are small, less than 1 mm in diameter

Giant beardworms that live in deepwater hydrothermal vents are 3 m long and 5 cm in diameter


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Most live in mud on ocean floor at depths of 100 to 10,000 m

Sessile animals that secrete and live in long chitinous tubes

Tubes have general upright orientation in bottom sediments

Tubes are generally three or four times the length of the animal



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Body Long cylindrical body covered with cuticle

Divided into a short anterior forepart, a long slender trunk, and a small, segmented opisthosoma

Tentacles located on a cephalic lobe Hollow extensions of the coelom and bear minute

pinnules

Lie parallel to one another, enclosing a cylindrical intertentacular space



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Internal body

No mouth or digestive tract

Mode of digestion puzzling

Nutrients such as glucose and amino acids absorbed from seawater through pinnules and microvilli of tentacles

Most energy derived from a mutualistic relationship with chemoautrophic bacteria that oxidizes hydrogen sulfide

Trophosome, derived embryonically from midgut, houses the bacteria



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Reproduction and Development Sexes are separate

Pair of gonads and gonoducts in trunk section

Research suggests that cleavage is unequal and atypical Appears to be spiral

Coelom formed by schizocoely

Embryo Worm-shaped and ciliated

Poor swimmer

Probably carried by water currents until it settles



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Clade Clitellata

Class Oligochaeta and Class Hirudinida

Form reproductive structure called a clitellum Ring of secretory cells found in a band around the

body

Permanent in oligochaetes but visible only during reproductive season in leeches

Members are derived annelids that lack parapodia

Hermaphroditic (monoecious) animals that exhibit direct development

Young develop inside a cocoon secreted by the clitellum, and emerge as small worms


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Class Oligochaeta Diversity

Over 3000 species

Occur in habitats from soil to freshwater

Few are marine or parasitic

Nearly all bear setae

Fewer in number than in polychaetes

Form and Function

Sometimes called “night crawlers”

Burrow in moist rich soil and usually live in

branched interconnected tunnels

Clade Clitellata


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Damp, rainy weather: Remain near surface

Dry weather: Burrow deep underground and go dormant coiled in a slime chamber

Peristaltic movements Contractions of circular muscles in the anterior end

lengthen the body, pushing the anterior end forward where it anchors

Anchoring is accomplished by contraction of the longitudinal muscles in forward segments

Causes the segments to become short and wide, pushing against the burrow

Clade Clitellata


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Setae Bristlelike rods set in a sac and moved by tiny

muscles

Project outward through small pores in cuticle

Aid anchoring by digging into walls of burrow

Nutrition Scavengers, feeding on decayed organic matter,

leaves, refuse, etc.

Food moistened by mouth and drawn in by a sucking action of muscular pharynx

Calcium in soil leads to high blood Ca+2 Calciferous glands along the esophagus reduce

calcium ion concentration in the blood

Clade Clitellata


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Food is stored in a thin-walled crop

Muscular gizzard grinds food into small pieces

Digestion and absorption occur in intestine

Typhlosole increases surface area

Chloragogen tissue surrounds the intestine

Synthesizes glycogen and fat

Cells full of fat float free in the coelom as eleocytes

Also function in excretion

Clade Clitellata


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Circulation and Respiration Coelomic fluid and blood transport food, wastes,

and respiratory gases

Blood circulates in a closed system with five main trunks running lengthwise in the body

Dorsal vessel contains valves and functions as a true heart

Pumps blood anteriorly into 5 pairs of aortic arches

Aortic arches ensure steady pressure in ventral vessel

Clade Clitellata


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Ventral vessel serves as an aorta, delivering blood to body walls, nephridia, and digestive tract

Blood contains colorless ameboid cells and dissolved hemoglobin

No special gaseous exchange organs

Gas exchanged across body surface

Clade Clitellata


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Excretion Each somite, except the 1st three and terminal

one, have a pair of metanephridia

Each unit occupies parts of two adjacent somites

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 nephridiopore

Wastes from both the coelom and the blood capillary beds are discharged

Aquatic oligochaetes excrete toxic ammonia

Clade Clitellata


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Terrestrial worms excrete less toxic urea

Chloragogen cells that break off and enter

nephridia produce urea and ammonia

Salts pass across the integument, apparently by

active transport

Clade Clitellata


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Nervous System and Sense Organs Central nervous system and peripheral nerves

Pair of cerebral ganglia connect around the pharynx to the ganglia of the ventral nerve cord

Fused ganglia in each somite contain both sensory and motor fibers

Neurosecretory cells in brain and ganglia secrete neurohormones

Regulate reproduction, secondary sex characteristics, and regeneration

One or more giant axons located in ventral nerve cord increase rate of conduction and stimulate contractions of muscles in many segments

Clade Clitellata


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Lack eyes but have many photoreceptors in the

epidermis

Free nerve endings in tegument are probably

tactile structures

Clade Clitellata


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General Behavior

Avoid bright light (negative phototaxis)

Chemical stimuli are important in locating food

Limited learning ability

Primarily trial-and-error learning

Clade Clitellata


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Reproduction and Development Monoecious

In Lumbricus, reproductive structures are located in somites 9 through 15

Sperm produced by testes mature in seminal vesicles and pass into sperm ducts

Eggs are discharged by ovaries into coelomic cavity

Ciliated funnels transport them outside

Two pairs of seminal receptacles receive and store sperm

Clade Clitellata


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Mate at night during warm, moist weather

Aligning in opposite directions, ventral surfaces together

Mucus secreted by clitellum holds worms together

Sperm from each worm are transported to the seminal receptacles of the other along seminal grooves

After mutual copulation, each worm secretes a mucus tube and chitinous band to form a cocoon

Cocoon passes forward and eggs, albumin, and sperm are added

Fertilization and embryogenesis occur in cocoon

Young worms emerge from cocoon

Clade Clitellata


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Class Hirudinida: Leeches

Divided into 3 orders Hirudinea (‘true” leeches)

Acanthobdellidae

Branchiobdellidae

Hirudinea 34 segments, lack setae and possess anterior and

posterior suckers

Acanthobdellidae 27 segments, setae only present on the first five

segments, and have a posterior sucker

Branchiobdellidae 14 or 15 segments, no setae, and an anterior sucker

Commensal or parasitic on crayfish

Clade Clitellata


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Diversity Most freshwater, few marine or live in moist

terrestrial environments

More common in the tropics temperate zones

Vary in color: black, brown, red, and olive green

Most are flattened

Some carnivores feeding on small invertebrates

Others are temporary or permanent parasites

Hermaphroditic

Form a clitellum during breeding season

Secretes a cocoon for reception of eggs

Clade Clitellata


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Form and Function Usually have a fixed number of segments

Appear to have more due to superficial annuli

Lack distinct coelomic compartments

No septa

In most, coelomic cavity filled with connective tissue and spaces (lacunae)

Lacunae channels may serve as auxiliary circulatory system

No setae

Developed suckers for attachment

Gut specialized for storage of large quantities of blood

Most use suckers to attach and “inchworm” along surfaces

Clade Clitellata


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Nutrition Not all are parasites, many are predaceous

Freshwater leeches have a proboscis for ingesting small invertebrates as well as to suck blood

Some terrestrial leeches feed on insect larvae, earthworms, and slugs

Other terrestrial leeches climb trees or bushes to reach warm-blooded vertebrates such as baby birds

Most are fluid feeders that prefer tissue fluids and blood pumped from open wounds

Some parasitic leeches leave a host during breeding season

Fish leeches may remain on a host

Clade Clitellata


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Respiration and Excretion

Some fish leeches have gills

All other leeches exchange gases across

epidermis

10 to 17 pairs of nephridia

Coelomocytes and other special cells may assist

in excretion

Clade Clitellata


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Nervous and Sensory Systems Two “brains”

Anterior fused ganglia form a ring around the pharynx

Seven pairs of posterior fused ganglia

21 pairs of segmental ganglia in between along a double nerve cord

Epidermis contains free sensory nerve endings and photoreceptor cells

Row of sensillae in central annulus of each segment.

Pigment-cup ocelli are present

Clade Clitellata


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Reproduction

Hermaphroditic and practice cross-fertilization

Sperm transferred by hypodermic impregnation

Clitellum secretes cocoon to receive sperm and egg

Cocoons are buried in mud or damp soil

Development is similar to that of oligochaetes

Clade Clitellata


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Circulation Coelom reduced by invasion of connective tissue

and chloragogen tissue

Forms system of coelomic sinuses and channels

Some have a typical oligochaete circulatory system

Coelomic system is auxiliary

Some lack blood vessels and coelomic sinuses serve as only vascular system

Clade Clitellata


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Classification Class Polychaeta

Class Oligochaeta

Class Hirudinida

Clade Clitellata


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Phylum Echiura

Diversity Approximately 140 species of marine worms

that burrow into mud or sand

Live in empty snail shells or sand-dollar tests, or rocky crevices

Found in all oceans

Length varies from a few millimeters to 40 or 50 cm


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Form and Function Sausage-shaped

Inextensible proboscis anterior to the mouth

Often called spoon worms”

Simple nervous system with a ventral nerve running length of the body

Ciliated groove on the proboscis allows them to gather detritus over the mud while lying buried

Muscular body wall is covered by a cuticle and epidermis which may be smooth or covered by papillae

Phylum Echiura


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Large coelom

Digestive tract long and coiled

Pair of anal sacs may serve an excretory and osmoregulatory function

Most have a closed circulatory system with colorless blood Hemoglobin found in certain cells and in coelomic

corpuscles

Respiration probably occurs in hindgut which is continually filled and emptied by cloacal irrigation

Phylum Echiura


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Sexes are separate

Gonads produced by special regions in

peritoneum in each sex

Fertilization usually external

Early cleavage and trochophore stages

similar to annelids

Phylum Echiura


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Phylum Sipuncula

Diversity Approximately 250 species of benthic marine

worms

Sedentary, living in burrows of mud or sand, snail shells, coral crevices, or among vegetation

More than ½ restricted to tropical zones

Some are tiny, slender worms, but most range from 3 to 10 cm in length

Some are known as “peanut worms” because when disturbed, they contract to a peanut shape


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Form and Function No segmentation or setae

Slender, retractable introvert or proboscis at

anterior end

Walls of the trunk are muscular

Phylum Sipuncula


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Nutrition Little known about feeding habits

Some appear to be detritivores and others suspension feeders

Some nutrition may come from dissolved organic matter in the surrounding water

From burrow or hiding place, they extend tentacles to explore and feed

Collected organic matter moved from mucus on tentacles to mouth by ciliary action

Retractor muscles retracts the introvert to allow tubular compensation sacs along the esophagus to accept fluid from tentacles

Large fluid-filled coelom

Digestive tract is U-shaped

Phylum Sipuncula


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Respiration Lack a circulatory and respiratory system

Gas exchange appears to occur across the introvert and tentacles

Nervous and Sensory Systems Bilobed cerebral ganglion behind tentacles

Ventral cord extends the length of body

Phylum Sipuncula


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Reproduction Sexes are separate

Sex organs develop seasonally within the connective tissue covering the origins of the retractor muscles

Sex cells are released through the nephridia.

Asexual reproduction occurs by transverse fission

Phylum Sipuncula


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Evolutionary Significance of Metamerism

Origins of Metamerism and the Coelom No satisfactory explanation for origins of

metamerism and coelom has gained acceptance

Coelom may have been advantageous as a hydrostatic skeleton

Coelomic fluid would have acted as a circulatory fluid and reduce need for flame cells everywhere

Coelom could store gametes for timed release

Would require nervous and endocrine control

Unlikely that segmentation is homologous among annelids, arthropods, and chordates

Current evidence supports the hypothesis that segmentation arose independently multiple times


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Selective advantage of a segmented body for annelids appears to lie in the efficiency of burrowing

However, does not explain segmentation in arthropods given the rigidity of the exoskeleton

Evolutionary Significance of Metamerism


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Phylogeny and Adaptive Diversification

Molluscs and annelids share many developmental features so are presumed to be closely related

However, shared features are likely to be a retained ancestral feature for lophotrochozoan protostomes

Pogonophorans and vestimentiferans were once placed in Annelida but are now in clade Siboglinidae

Molecular analyses place sipunculids and echiurans closely related to the annelids


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Some biologists place echiurans within

Annelida because they have serially repeated

structures

One recently developed phylogenetic tree

place echiurans near capitellid polychaetes

because both dwell in sediments



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With more studies, Echiura, like Pogonophora, may no longer be a valid phylum

Placement of Sipuncula is contentious

Members are not metameric and lack setae

Larval development similar to annelids, molluscs, and echiurans

Molecular data may clarify position within Lophotrochozoa

Presently, depicted as the sister taxon to a clade of annelids and echiurans

Polychaeta is a paraphyletic group



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Adaptive Diversification

Oligochaetes are constrained by terrestrial soil

environment

Polychaetes inhabit a wide range of habitats

Septal arrangement with fluid-filled compartments

has been varied for precise movements

Feeding adaptations vary widely, from chitinous

jaws to specialized tentacles

Leeches have developed both parasitic and

predatory adaptations


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Annelids and Allied Taxa - Unicamdocenti.unicam.it/tmp/2596.pdf · Annelids and Allied Taxa . ... annelids, and in vertebrates, ... Excretion Excretory organs