PHYLUM ANNELIDA

RING WORMS

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P o l y c h a e t a Hirudinea

Oligochaeta

Phylum Annelida-Ring worms

• marine, fresh water & moist land habitats • Protostomian coelomates • Vermiform, worm like • Soft bodied • Round in cross section • Metameric segmentation with septa between segments: replication of organs

• Locomotion is a function of circular and longitudinal muscle contraction in individual segments or group of segments and prevent deformation

• Oblique muscles: maintain body turgor and operate parapodia • move by peristalsis-a type of locomotion produced by rhythmic waves of

muscle contractions passing from head to tail • hydrostatic skeleton by coelomic fluid in individual segments • flexible thin outer body wall: permeable for gas • Gas exchange through moist epidermis that forms the body wall with or without

cuticle secreted by epidermis.

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Metanephridia waste removal

• Little kidney

• Each segment contains 2 nephridia

• Each opens at both ends with 2 opinings

• ciliated funnels called nephrostomes that remove wastes from the blood and coelomic fluids

Parapodia • Parapodia present in some groups • One or many pairs of chitinous bristles

called setae (chaetae). • Functions:

– Morphology needs for classification – Highly vasculized- exchange of gases – Locomotory function

– Some species with overlapping elytra

protective plates

Phylum Annelida-Digestive system

• Complete compartmentalized

digestive system in most groups: mouth, pharynx, esophagus, intestine, rectum, anus.

Pharynx is associated with salivary glands (that secrete hirudin anticoagulant in Hirudinea),

Esophagus may lead to a crop then gizzard and associated with calciferous glands- for control of Ca ion concentration- in oligochaeta,

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Phylum Annelida- Circulatory system

• Closed • Consists of dorsal blood vessel (carrying blood

towards anterior end) • ventral blood vessel (carrying blood towards

posterior end) • capillaries that connect the two vessels • Contraction of blood vessels and valves ensure

circulation and movement of blood in one direction

• Anterior blood vessels (5 pairs) connecting between dorsal and ventral blood vessels may be enlarged to act as hearts

• Blood pigment mostly hemoglobin but some polychaeta have chlorocruorin or hemoerythrin as blood pigments (both Fe containing pigments)

• Circulatory system much reduced or absent in Hirudinea with coelomic fluid used for circulation

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Phylum Annelida- Nervous system

• Cerebral ganglia (brain) located above buccal cavity

• connected to a subpharyngeal ganglion

• and a series of ventral segmental ganglia running throughout body

Sense organs include:

touch receptors (tentacles and palps)

statocysts,

photoreceptors

mechanoceptors (vibration)

chemoreceptors that include ciliated slit nuchal organs in head of most polychaetes.

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Classification of Annelida: Class I. Polychaeta (many setae)

• marine mostly benthic habitat

• About 70% of all annelids

• Possess a minimum of one pair of eyes and anterior sensory appendages

• Paired parapodia carrying acicula (chitinous support rods) and many siliceous, chitinous or calcareous setae,

• Parapodia: paired outfoldings; function as locomotive and gas exchange; Some species with overlapping elytra protective plates;

• Septa between anterior segments may be absent or perforated especially in active burrowing spp. to allow this part to function as proboscis (penetrating organ into sediment);

• Muscles include circular, longitudinal and oblique muscles;

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A. Fireworm (Eurythoe spp);

B. Fan or Feather worm;

C. Calcarious tubes of Sedentary

polychaetes.

Perinereis

Ophryotrocha

Acrocirrus

Nereis

Polychaeta Reproduction & Life Cycle • Asexual:

– fragmentation & regeneration of missing parts – budding

• Sexual: – worms are dioecious with gametes produced by special peritoneal tissue in

certain segments or most segments – fertilization external in water

Some polychaetes undergo epitoky in preparation for sexual reproduction. Male or female individual (atoke) in a tube or burrow develops into or buds at its posterior end one or more epitokes (specialized for sexual reproduction); epitokes detach, swim away and commingle with other epitokes and discharge sperms or eggs for external fertilization.

• Life cycle – mostly as follows: fertilized egg---trochophore larva (wheel bearer), active

swimmer---adult. – In some spp, fertilized eggs develop into juveniles within gelatinous mass or

brooding chambers that grow continuously to reach adult stage.

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Reproduction in Annelids

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Myrianida

epitoke formation Trochophore larva

Asexual reproduction

by budding

Polychaeta Groups I (Two groups based on mode of living)

Group Errantia • Errant, active and mobile;

• Well developed parapodia for active oar-like locomotion and eel-like movement on substratum;

• Well developed head appendages including palps, tentacles, and toothed or jawed protrusible pharynx;

• Mostly carnivorous but some are suspension or detritus feeders and others are omnivores;

• Most spp surface dwellers but some burrow & live within substrates.

• Examples: Nereis, Harmothoe (have overlapping elytra),

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Errant Polychaetes

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Polychaeta Groups

Group Sedentaria • Sedentary living within simple

burrows or well constructed rigid and protective tubes within sediment

• Parapodia reduced, modified into fan like shape for water movement

• Suspension or deposit feeders.

• Examples: – Chaetopterus (in tube with two

openings),

– Arenicola (with gills),

– Sabella (with fan like anterior filaments projecting from the tube)

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Classification of Annelida: Class II. Clitellata

• Mostly in fresh water and moist terrestrial habitats with few marine species

• Clitellum (specialized glandular epidermis surrounding certain segments and functions in secreting mucus to help transfer sperms between reciprocally copulating worms, secreting a cocoon where embryos develop, and secreting albumin that embryos feed upon);

• No parapodia

• No sensory appendages

• Gas exchange by diffusion across moist body wall

• Monoecious with permanent gonads (ovaries and testes) in certain segments – Cross fertilization as a rule

– No trochophore larva in life cycle

– instead embryos develop within cocoons into juveniles that grow steadily into adults.

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Clitellata Subclasses:

Subclass Oligochaeta & Subclass Hirudinea

Subclass Oligochaeta • Mostly in freshwater and moist terrestrial habitats • About 6.5% are marine • Deposit feeders • Parapodia are lacking • Few setae in two sets per segment • Locomotion through peristaltic waves (continuous series of localized

contractions and relaxations of circular and longitudinal muscles) that help in burrowing and moving.

• Reproduction: Asexual & sexual: -Asexual: by transverse fragmentation and regeneration of missing parts, or

by parthenogenesis (eggs develop without fertilization). -Sexual by reciprocal: alignment of two hermaphroditic worms transfer of sperms for external cross fertilization of eggs embryos develop in cocoon secreted by clitellum, development of juveniles which grow into adults. • Example: Lumbricus (earthworm).

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Clitellata Subclasses: Subclass Oligochaeta & Subclass Hirudinea

Subclass Hirudinea (leeches) • mostly in freshwater or terrestrial moist habitats, few spp. in marine

habitat

• lack setae

• Lack head appendages

• Locomotion by looping aided by having anterior and posterior suckers, longitudinal and circular muscles.

• No septa between segments although superficial and deep annuli distinct

• coelomic space largely filled with mesenchyme

• Reduced or no circulatory system, coelomic fluid function as circulatory medium;

•

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Connective mesenchyme tissue

Clitellata Subclasses: Subclass Oligochaeta & Subclass Hirudinea

Subclass Hirudinea (leeches) (Continued) • Mostly spp. (75%) ectoparasitic blood suckers and rest

are predators on invertebrates; Blood suckers are mostly equipped with 3-toothed jaws or protrusible proboscis to feed on blood of vertebrates or invertebrates – They secrete anticoagulant (hirudin) – local anesthetic – antibiotic substances

• Medical use is blood letting

• Reproduction: Only sexual by reciprocal copulation of two hermaphroditic worms

Cross fertilization internal through direct transfer of sperms or jabbing spermatophores through body wall.

Development occurs in cocoon where embryos develop into juveniles that grow into adults and leave cocoon.

• Example: Hirudo medicinalis (medicinal leech).

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