Zoology ch13annelida

Phylum AnnelidaChapter 13

At the time of the November full moon on islands near Samoa in the South Pacific, people rush about preparing for one of their biggest yearly feast. In just one week, the sea will yield a harvest that can be scooped up in nets and buckets. Worms by the millions transform the ocean into what one writer called “vermicelli soup!” Celebrants gorge themselves on worms that have been cooked or wrapped in breadfruit leaves.

Samoan Palolo worm

Stock photo of hunters with powerful lights used to attract more palolo worm, Palola siciliensis, American Samoa, Pacific

Samoan Palolo worm

Image of palolo worms, Palola siciliensi, in water column after emerging from their burrows on night of the spawning, Tutuila Island, American Samoa, Pacific Ocean

Phylum Annelida• Class Polychaeta

•Marine worms

• Class Clitellata–Subclass Oligochaeta-

•Soiling building worms–Subclass Hirudinea-

•Predatory leeches

Phylum AnnelidaCharacteristics

–Metamerism - segmented body

–Bilateral–Protostome–Wormlike–Epidermal Setae–Closed Circulatory System–Dorsal ganglia and ventral nerve cord

–Metanephridia or protonephridia

Metamerism and Tagmatization

Metamerism – segmental arrangment of body parts.

Each segment has:excretory, nervous, and circulatory structures

Advantages of Metamerism:

• 1. Hydrostatic compartments- allows variety of locomotor and supportive functions like swimming, crawling, and burrowing.


• 2. Lessens the impact of injury- if few segments injured others can perform normal functions which increases the likelihood that the worm will survive.


• 3. Tagmatization- The specialization of body regions in a metameric animal

• permits the modification of certain regions of the body for specialized functions like feeding, locomotion, and reproduction.

Class Polychaeta

Class PolychaetaCharacteristics

• Marine• 5 – 10 cm in length

• 5,300 species• Largest of the annelid classes

Class Polychaeta

Live:• On the ocean floor• Under rocks and

shells• Within crevices of

coral reefs• Some can burrow

External Structure and Locomotion

Parapodia – lateral extensions supported by chitin

Setae – bristles secreted from the distal ends of the parapodia• Important for locomotion/ digging

Parapodium

Parapodia and Setae

Parapodia and Setae

Parapodia and SetaeSee them work in action


Prostomium- lobe that projects dorsally and anterior to mouth

contains: eyes, antennae, palps and nuchal organs

Nuchal organs: ciliated sensory pits which are chemoreceptors for food detection


Peristomium- first body segment; surrounds the mouth

Annelids also secrete a nonliving cuticle from the epidermis for protection

Prostomium and Peristomium

Prostomium and Peristomium

The prostomium (right) has two antennae and two large, two-segmented palps. The peristomium (segment behind the prostomium) has 4 pairs of tentacular cirri. This photo is a 3d composite made from a series of photos using a Keyence digital microscope. Photo by Dave Cowles, July 2009

Feeding and the Digestive System

Digestive tract is a straight tube:Pharynx- when everted can form a

proboscis (See it)Crop- storage sacGizzard- grindingIntestine-long and straight


• Polychaeta can be:–Predatory-usually burrow or live in coral crevices; some can have poison glands

–Herbivores–Scavangers–Filter feeders- tube dwelling (see them feed)

Examples of different worms and how they feed

Selective deposit feeders with tentacles.Filter feeders with radioles.


Elimination of waste for tube dwellers:If open ends: wastes carried away by water circulating in tube.

If closed end: then the worms either turns around in tube OR uses ciliary tracts along body wall to carry feces out.


• Polychaetes that inhabit substrates rich in dissolved organic molecules can absorb as much as 20% to 40% of their food across their body walls (very unusual among animals)

Gas Exchange and Circulation

• Respiratory gases diffuse across body wall and parapodia (increase surface area)

• Closed circulatory system• Oxygen is carried by molecules called

respiratory pigments- blood colorless, green or red

Gas Exchange and Circulation

• Dosal aorta- propels blood from rear (posterior) to front

• Ventral aorta- propels blood from front to rear

• With Capillaries between the two aortas

Nervous and Sensory• A pair of Suprapharyngeal ganglia-controls motor and sensory functions; feeding and forward movement.

Connects to

• A pair of Subpharyngeal ganglia- mediates locomotor functions required for coordination of distant segments.

By

• Circumpharyngeal connectives-run dorsoventrally along the pharynx

Nervous and Sensory

• Ventral nerve cord with paired segmental ganglia in each segment; allows escape response of segments

• Segmental ganglia- coordinate swimming and crawling movements in isolated segments.

• 2–4 pairs of eyes

Suprapharyngeal ganglia

Circumpharyngeal connectives

Subpharyngeal ganglia

Segmental ganglia

Excretion

• Excrete ammonia• Most of the excretory organs

in annelids are active in regulating water and ion balances

• Nephridia – excretory organs in annelids

Two types of Nephridia

• Protonephridia- tubule with a closed bulb at one end and a connection to the outside of the body at the other end.

• Metanephridia- open ciliated funnel (nephrostome) that projects through an anterior septum into the coelom of an adjacent segment

Excretion

Osmoregulation and waste removal

Metanephridia with nephrostomes open to coelomic fluid.

Lead to nephridioducts which reabsorb wanted materials.

Waste released through nephridiopore.

• Nephridia

Reproduction and Regeneration

•All polychaetes can regenerate lost segments

•May have natural break points if grabbed by predators (process called autotomy)

•Some reproduce by fission or budding


• Most sexually reproduce• Most are dioecious • External fertilization and

trochophore larvae• Swarming occurs in some

species, where large numbers of individuals join together to release sperm and/or eggs


• Very few species copulate (most external fertilization)

• A unique, weird, kinky copulatory habit has been reported in Platynereis megalops from Woods Hole, Massachusetts. Read page 216 for the details. . .


• Epitoky- formation of a reproductive individual (an epitoke) that differs from the nonreproductive form of the species (an atoke).

• Epitoke- body modified into 2 body regions; anterior segments-normal maintenance; posterior segments-enlarged & filled with gametes

• Example: Samoan palolo worm swarm

In some species the epitoke breaks free from the atoke, which stays in the burrow

The common clam worm Nereis succineain its atoke form (above) and epitoke form (below). Especially note the enlarged parapodia on the epitoke; it uses these to swim and release eggs or sperm.

In others, the epitoke is formed as part of the body, and the whole animal leaves

to mate

Epitoke on the left, atoke on the right

3 Advantages of Swarming Epitokes

1. Nonreproductive individuals remain safe below the surface waters; predators cannot devastate an entire population.


2. External fertilization requires individuals to be ready at the same time. Swarming ensures large numbers of individuals are in the right place at the right time.


3. Swarming of vast numbers of individuals for brief periods provide a banquet for predators. But because it is such a BREIF period, predators can only eat so much with respect to the limits of their normal diets. Predators can dine gluttonously and still leave epitokes that will yield the next generation of animals.

Class Clitellata

Giant Blue EarthwormTerriswalkeris terraereginae

mucin it releases is luminescent Lives in rainforest in Australia

Earthworms and Leeches

Class Clitellata Earthworms and Leeches

•Have a clitellumused in cocoon formation

•All are monoecious

•Have few or no setae

Class Clitellata

Subclass Oligochaeta-

Soiling building worms(earthworms)

Subclass Hirudinea-Predatory leeches

Subclass Oligochaeta

• 3,000 species• Freshwater and terrestrial

habitats throughout the world (some marine)

• Aquatic species live in shallow water; burrow in mud and debris

• Terrestrial species live in soils with high organic content

So you want to see the biggest earthworm in

the world? Go to Australia or sit back and

watch this. . .

Giant Gippsland Earthworm


External Features• Have setae but fewer• Lack parapodia- get in the way

because of burrowing

• Prostomium- lacks sensory appendages

External Features

• Clitellum- secretes mucus during copulation and forms a cocoon (girdle-like structure)

Locomotion• Have both circular and longitudinal muscles• Move by antagonistic contractions of these

muscles• Bulging and elongating body segments in

waves cause the worm to move forward• Small setae help anchor the worm• Small conical prostomium acts like a wedge

while burrowing, and soil is swallowed (important for decomposition)

Feeding and Digestive System

• Scavengers-fallen and decaying vegetation

• Mouth->muscular pharynx->esophagus• Esophagus expanded form of stomach,

crop ( thin-walled storage structure), gizzard (muscular grinding structure).

• Calciferous glands-evaginations of esophagus wall that rids the body of excess calcium absorbed by food; regulates pH

Feeding and Digestive System

• Intestine-principle site of digestion and absorption

• Anus


• Gas Exchange and Circulation: same as polychaetes

• Nervous and Sensory: same as polychaetes but lack well-developed eyes

Excretion

• Oligochaetes use metanephridia for excretion of ammonia and urea and for ion and water regulation.

• Chloragogen tissue- acts like a liver for amino acid metabolism (deaminates amino acids into ammonia and urea); excess carbohydrates converts into glycogen and water

Reproduction

•Monoecious•Reproduce sexually via reciprocal fertilization-both worms exchange sperm (can last 2-3 hours!)•Cocoon of mucous and chitinous materials produced by clitellum•Eggs, sperm, and food (albumen) deposited in cocoon where fertilization takes place•Young worms hatch from cocoon (no larvae)

Reproduction

Eggs

Figure 17.17

Reproduction

• Freshwater oligochaetes can reproduce asexually which is usually followed by the regeneration of missing segements.

Subclass Hirudinea• 500 species• Mostly freshwater but some marine

and terrestrial• Prey on small invertebrates or feed

on the body fluids of vertebrates.

External Structures• Lack parapodia and head

appendages• Leeches are dorsoventrally

flattened and tapered anteriorly

• Anterior and posterior segments have suckers

External Structures

• Have 34 segments

Locomotion• Have lost metameric partitioning,

resulting in single body cavity• Coelomic sinuses replace blood

vessels in most leeches • Complex musculature (four types

of muscles)• Move in looping motion or swim with

undulations

Locomotion• Inchworm-like

crawling.• Relatively few

but large neurons.

• Extremely sensitive to temperature and vibration.

Feeding

• Feed on body fluids, blood of vertebrates, or entire bodies of invertebrates

• Ectoparasites

Feeding• Mouth with sucker-3 chitinous jawsor proboscis

Feeding

• Have an anticoagulant and an anesthetic in saliva

• Anticoagulant: Hirudin- prevents blood from clotting

Feeding

• Pharynx-> esophagus-> large stomach with lateral cecea (increase body mass 2-10 times)-> short intestine-> anus

Figure 17.20

Subclass Hirudinea•Nervous System

Temperature senses

•Excretion10 to 17 nephridia

•ReproductionMonoeciousReproduce sexually ONLYClitellum present only in the spring

Subclass Hirudinea

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