Phylum Annelida
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Animals in the phylum Annelida include a variety of earthworms, leeches, and marine
polychaetes. Annelids can be freshwater, marine or terrestrial. Some are adapted to parasitic
living. They are typically elongate, wormlike animals that are circular in cross section and
have muscular body walls. The most distinguishing characteristic that sets them apart from
other wormlike creatures is their segmentation. They are often referred to collectively as the
"segmented worms." This repetition of body parts, also called metamerism (me-ta'me-ri'sum;
Gr. meta, between, + meros, part), not only is external but also is seen internally in the serial
repetition of body organs. Development of segmentation is of significance in the general
evolutionary trend toward specialization, for along with segmentation comes the opportunity for
segments to become specialized for certain functions. Such specialization is not as noticeable
in annelids as in arthropods, but the introduction of metamerism coincided with the rapid
evolution of advanced organization seen in arthropods and chordates, the only other phyla
emphasizing segmentation.
Division of the coelomic cavity into fluid-filled compartments also has increased the usefulness
of hydrostatic pressure in the locomotion of annelids. By shifting coelomic fluid from one
compartment to another through perforations in the dividing septa, differential turgor can be
effected, permitting a preciseness of body movement not possible in pseudocoelomates. The
coordination between their well-developed neuromuscular system and more efficient
hydrostatic skeleton makes annelids proficient in swimming, creeping, and burrowing.
Annelids have a complete mouth-to-anus digestive tract with muscular walls, so that digestive
tract movements are independent of body movements. There is a well-developed closed
circulatory system with pumping vessels, a high degree of cephalization, and an excretory
system of nephridia. Some annelids have respiratory organs.
The annelids are true coelomates which form their coelom by a separation or splitting o
mesoderm after it is laid down between the primitive gut and the body wall. Animals which
form a coelom in this manner are called schizocoelomates. The schizocoelomates are usually
protostomes. The schizocoelomate group includes the phyla Annelida, Mollusca and
Arthropoda, among others.
Body
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Plan Features Retained by Annelids (seen in previously studied phyla):
bilateral symmetry
eucoelomic
high degree of cephalization = well developed head with sensory organs
triploblastic structure (endoderm, mesoderm and ectoderm)
all organ systems are present
complete digestive system
excretory system
Body Plan Features Characteristics of Members of the Phylum Annelida
1. segmentation (metamerism) both internal and external
2. highly developed true coelomic cavity
3. more complex closed circulatory system
4. fleshy parapodia in some members is likely an example of evolutionary convergence to
appendages and specialized gills seen in arthropods
5. well developed nephridia in most segments for excretion
6. some have respiratory organs
Phylum Annelida
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Phylum Annelida
Annelids are members of the Superphylum Lophotrochozoa
Classification
Class Polychaeta (pol'e-ke'ta) (Gr. polys, many, + chaiti?, long hair). About 5300 species.
Segmented inside and out; parapodia with many setae; distinct head with eyes, palps, and
tentacles; no clitellum; separate sexes, trochophore larva usually present; mostly marine.
Examples: Nereis, Chaetopterus.
Class Oligochaeta (ol'i-go-ke'ta) (Gr. oligos, few, + chaiti'!, long hair). About 3100 species. Body
segmented inside and out; number of segments variable; clitellum present; few setae; no
parapodia, head poorly developed; coelom spacious and usually divided by intersegmental
septa; direct development; chiefly terrestrial and freshwater. Examples: Lumbricus, Tubifex.
Class Hirudinae (hir'u-din' ea) (1. hirudo, leech, + ea, characterized by). About 500 species.
Segments 33 or 34 in number, with many annuli; clitellum present; anterior and posterior
suckers; setae absent (except Acanthobdella); parapodia absent; coelom closely packed with
connective tissue and muscle; terrestrial, freshwater, and marine. Examples: Hirudo,
Placobdella.
Average Sizes
¾ millimeters to meters
Life Span
¾ months to years
Symmetry & Body Plan
¾ bilateral symmetry with vermiform (worm-like) shape
¾ body cavity is a true coelom (eucoelomate)
¾ metameric (segmentation)
¾ cephalized
Development & Coelom Formation
¾ true tissues
¾ eucoelomate
¾ triploblastic body plan with endoderm, mesoderm, and ectoderm
¾ spiral cleavage
¾ mosaic (determinate) development similar to mollusks
¾ metamerism more primitive than arthropods as there are few differences between
segments
Type of Skeleton
¾ hydrostatic skeleton via the fluid filled and segmented coelom
¾ outer secreted cuticle from epidermis can form tubes in some sedentary species
Appendage Types
¾ parapodia with many setae in polychaetes; no parapodia in oligochaetes or leeches
Phylum Annelida
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Basic
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Form & Function
segmented (metameric) bilateral body with circular and longitudinal muscle layers
prostomium = head portion
pygidium = terminal posterior portion
new segments differentiate just in front of the pyugidium so the oldest segments
are at the anterior end and the youngest segments are at the posterior end
¾ outer transparent moist cuticle is secreted by the epithelium
¾ tubes of protein (leathery) or calcium secreted by some polychaetes
Specialized Cell Types/Organs
¾ chitinous setae often present; setae absent in leeches; greater number of setae in
polychaetes as compared to oligochaetes
¾ parapodia in polychaetes with more dorsal notopodium and ventral neuropodium
¾ modified feathery radiole arms in some tube worms
¾ modified parapodia form fans for water movement in parchment worms
¾ nephrostome = ciliated funnel that connects excretory system between segments
¾ clitellum in oligochaetes and leeches which forms a cocoon where egg fertilization and
development occurs
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“Organ Systems”
Integumentary System (skin)
¾ cellular epidermis that secretes a
¾ thickened, non-living transparent cuticle
¾ cilia present in many forms
¾ chitinous setae often present; setae absent in leeches; greater number of setae in
polychaetes as compared to oligochaetes
Muscular System
¾ both longitudinal and circular muscle fibers in body wall; peristalsis
¾ special muscles attached to setae to move setae
¾ in polychaetes, oblique muscle groups run into the parapodia for movement
Circulatory System
¾ closed circulatory system that is segmentally arranged with aortic arches (“hearts”)
and ventral and dorsal blood vessels for circulating blood
¾ blood contains respiratory pigments including hemoglobin
Respiratory System (gas exchange)
¾ gas exchange by simple diffusion = cutaneous respiration and/or
¾ gills in some polychaetes and/or
¾ parapodia in polychaetes act as gills = example of convergent evolution to appendages
and specialized gills seen in arthropods
Nervous System
¾ cerebral ganglia with two fused ventral nerve cords and various ganglia similar to
arthropods
¾ leeches have an anterior brain of six pairs of fused ganglia and a posterior brain of
seven pairs of fused ganglia
Sensory Organs
¾ sensory tentacles, palps and eyespots in polychaetes, especially on prostomium
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Phylum Annelida
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Biological Sciences 102 – Animal Biology – Notes & Vocabulary
Digestive System (food/water supply)
¾ complete digestive system with mouth and anus, not segmentally arranged
¾ well muscularized pharynx in oligochaetes
¾ crop for storage, gizzard for grinding and typhlosole with increase surface area for
absorption in oligochaetes
¾ in oligochaetes, ingested soil may contain high levels of calcium; calciferous glands
along esophagus secrete calcium ions into gut to reduce calcium ion concentration and
maintain acid-base balance in blood
¾ in tube worm polychaetes, specialized feathery arms or radioles use ciliary action
and mucous to trap food particles of a digestible size
¾ leeches are parasitic, but also predaceous and scavenging on fluids and tissues
¾ terrestrial leeches feed on insect larvae, earthworms and slugs
Immune System
¾ ameobocytes found in blood
Excretory System (waste removal)
¾ well developed metanephridia in most segments that involves removal of wastes from
the blood and the coelom
¾ generally have metanephridia; some have protonephridia
¾ each metanephridium has a ciliated funnel or nephrostome that leads by a small
ciliated tubule through the septum in to the segment behind to a nephric tubule that
empties into a bladder and waste is excreted out a nephridiopore (opening)
¾ aquatic annelids secrete ammonia; terrestrial oligochaetes secrete less toxic urea
¾ osmoregulation occurs via the body surface and the nephridia
Reproduction
¾ monoecious in oligochaetes and leeches (Hirudinae) with internal fertilization and
direct development (no larval stage = juvenile worms look like mini adults)
¾ dioecious in polychaetes with external fertilization and formation of a
trochophore larva
¾ asexual = most highly organized animals capable of complete regeneration. This
ability varies considerably within the phylum. Evolutionarily, regenerative capability
has been lost in many species. Both anterior (tail to head) and posterior (head to tail)
segment regeneration has been shown in some species.
Mechanism/Mode of Locomotion
¾ peristalsis involving the alternating contractions of longitudinal and circular muscle
layers to change the shape of individual segments
¾ burrowing and creeping using peristalsis and/or parapodia
Specialized Defenses/Toxin/Poisons
¾ irritating toxins released from the setae/parapodia of some polychaetes (fireworms)
Ecology & Adaptive Radiation
¾ marine (usually near coast), freshwater, terrestrial (soil) from polar regions to the topics
¾ from high mountains to deep seas
¾ great variation in feeding adaptations from the sucking pharynx of oligochaetes to the
sucker adaptations of leeches to the chitinous jaws of polychates and specialized
tentacles and radioles of filter feeding sedentary worms
No real Social Organization, Communication or Thermoregulation (=they are exothermic)