Phylum Annelida Characteristics

Phylum Annelida

• Defining characteristics:
  • One or more pairs of chitinous setae
  • Polychaetes can be divided into 2 general groups: Errantia (errant species) and Sedentaria
• General Annelid Characteristics:
  • All adult annelids except sipunculans possess at least one pair of chitinous setae
  • All are vermiform (worm-shaped) and longer than they are wide
  • Soft-bodies, circular in cross-section
  • Exhibit metameric segmentation or metamerism
  • Thin body wall can serve as a surface for gas exchange
  • Septa - thin sheets of mesodermally derived tissue (peritoneum)
  • Excretion occurs through nephridia (little kidneys)
  • Coelomic fluid is drawn into nephridium at the nephrostome
  • Function of nephridium: regulate water content of coelomic fluid, outlet for metabolic waste products, and discharge gametes and urine

Polychaeta

• Defining characteristics:
  • Paired lateral outfoldings of the body wall (parapodia)
  • 65% of annelids are polychaetes; nearly all live in saltwater
  • Have at least one pair of eyes and at least one pair of sensory appendages (tentacles) on the anterior-most part of the body
  • Parapodia are outgrowths of the body wall that function in gas exchange and locomotion
  • Setae - calcareous bristles protruding from each parapodium
• Reproduction:
  • Exclusively sexual; most are gonochoristic
  • Gametes are produced by peritoneal tissue
  • At least 6 adjacent segments are involved in gamete production
  • Some polychaetes undergo epitoky - morphological preparation for reproductive activity

Echiurans

• Defining characteristics:
  • Muscular organs (anal sacs) outpocketing from the rectum into the coelomic space, bearing numerous funnels that discharge coelomic fluid (and wastes) through the anus
  • Segmentation confined to small rear portion of animal (opisthosoma)
  • Anteriormost region of body bears cephalic lobe or "beard"
  • Trunk contains uninterrupted coelomic cavities and major organs within trunk: gonads and trophosome
• Representative species:
  • Urechis caupo
  • Bonellia viridis

Sipunculans

• Defining characteristics:
  • Anterior part of body forms an eversible and fully retractable introvert, with the mouth at its end
  • Multicellular bodies (urns) in the coelomic fluid, specialized for accumulating particulate wastes
  • Anterior tentacles connected to a series of muscular sacs (compensatory sacs) that pump fluid into the tentacles and store fluid when the tentacles retract
• Representative species:
  • Sipunculus nudus
  • Phascolosoma gouldi

Clitellata

• Defining characteristics:
  • Pronounced cylindrical glandular region of the body (clitellum) that plays important roles in reproduction
  • Permanent gonads
  • Hermaphroditic
• Representative species:
  • Lumbricus terrestris (common earthworm)
  • Tubifex (sludge-worms)

Leeches (Hirudinea)

• Defining characteristic:
  • Posterior sucker
• Representative species:
  • Hirudo medicinalis (medicinal leech)
  • Limnatis sp. (common carabao leech)### Regulation of Reproductive Cycle
• Regulation of reproductive cycle, body fluid osmotic concentration, migration of light-screening pigments in the eye, and movement of pigment granules within chromatophore cells leading to gradual changes in body color
• Tagmatization: specialization of groups of segments for highly specialized functions

Nerves and Muscles

• In vertebrates, each muscle fiber is innervated by a single neuron
• Strength of muscle contraction depends on the number of fibers contracting; number of fibers contracting depends on the number of axons fired
• In arthropod muscle, strength of contraction depends on the rate at which nerve impulses are delivered to the fibers
• Single muscle fiber may be innervated by 5 types of neurons
• Arthropod neuron may innervate a large number of muscle fibers

Exoskeleton

• Differences in exoskeleton between arthropods and molluscs:
  • Molluscan shell functions to protect soft parts within
  • In arthropods, exoskeleton functions as a locomotory skeleton
• Components of arthropod exoskeleton:
  • Epicuticle: waxy outermost layer, composed of firm lipoprotein layer and lipid layers, water-impermeable, and thin
  • Procuticle: bulk of exoskeleton, composed of polysaccharide chitin
  • Sclerotization: tanning of procuticle's protein component

The Hemocoel

• Coelom in arthropod is greatly reduced
• Hemocoel: main body cavity
• Cryptocyanin: molecule that evolved from hemocyanin, required in formation of exoskeleton among crustaceans

Molting

• Ecdysis: process of removing existing exoskeleton
• New cuticle is secreted before old one is shed
• Soft-bodied crabs rely on internal blood pressure in the hemocoel to maintain locomotory function
• Biomass: growth of tissue, continuous process

Circulatory System

• Blood leaves heart through closed vessels and enters heart directly from the hemocoel through perforations called ostia
• Circulatory system is open
• One of the diagnostic features of arthropoda: heart with ostia

Arthropod Visual System

• Ocellus: small cup with light-sensitive surface backed by light-absorbing pigment
• Compound eyes:
  • Can form images
  • Common in insects and crustaceans; may be present in addition to ocelli
  • Differences from human camera-type eye:
    • Many lenses
    • Focus of each lens cannot be varied
    • Fewer receptor cells to sample the image, which is upright rather than inverted
  • Ommatidium consists of:
    • Fixed-focus lens (cornea)
    • Underlying gelatinous crystalline cone
    • Series of cylindrical bodies (photoreceptors) containing light-sensitive pigment
    • Neural cartridge: cluster of neurons receiving information carried by photoreceptors and sending action potentials to optic ganglia### Myriapoda
• Defining characteristics:
  • Many species conserve water by being nocturnal
  • Chilopod head bears a single pair of antennae, pair of mandibles, and pair of first and second maxillae
  • Unique proboscis at the anterior end with an opening at its tip
• Body characteristics:
  • Not divided into distinct regions (tagmata)
  • Head followed by 15 or more leg-bearing segments; first pair is called maxillipeds
  • Some have repugnatorial glands on the ventral surface of each trunk segment
• Locomotion:
  • Slow-moving
  • Some produce silk; some burrow in soil (reduced legs and used properties of hydrostatic skeleton)
• Representative species:
  • Scutigera coleoptrata
  • Scolopendra gigantea
  • Hydroschendyla submarina

Diplopoda

• Defining characteristics:
  • Pair of segments fused; each segment (diplosegment) bears 2 pairs of legs, and 2 pairs of spiracles and ventral ganglia
• Body characteristics:
  • Primarily slow-moving deposit feeders; some carnivorous
  • In some millipedes, integument is impregnated with calcium salts, as in crustaceans; thus covering is more protective than centipedes
• Locomotion:
  • Cuticle not waxy
• Representative species:
  • Scutigerella sp.
  • Symphella sp.
  • Acladocricus sp.

Hexapoda

• Defining characteristics:
  • Possess both uniramous and biramous appendages
  • Includes 3 major groups: Myriapoda, Insecta, and Crustacea
• Body characteristics:
  • Six-legged arthropods
  • Most are insects; wingless hexapods (Entognatha); silverfish
• Respiratory system:
  • Gas exchange surfaces are internalized; achieved by means of a tracheal system

Insecta

• Defining characteristics:
  • Fusion of one pair of head appendages (the second maxillae) to form a lower lip (the labium)
  • Loss of abdominal appendages
• Body characteristics:
  • Most species are terrestrial; nearly one million species have been described
  • Halobates – ocean striders; live in the surface waters of the open ocean
• Flight:
  • Features that make flight possible:
    • Abundance of striated muscle
    • Muscle antagonism by means of lightweight, jointed skeleton
    • Small body size
    • Water-impermeable outer body covering, preventing dehydration
    • Efficient systems for gas exchange, nutrient storage, and distribution of nutrients to the musculature
    • Highly developed nervous and sensory systems for steering, navigating, and sensing wind direction

Crustacea

• Defining characteristics:
  • Head bears five pairs of appendages, including two pairs of antennae
  • Development includes a triangular larval form (the nauplius) bearing three pairs of appendages and a single medial eye
• Body characteristics:
  • 6 major classes
• Representative species:
  • Campodea staphylinus
  • Drosophila melanogaster
  • Manduca sexta
  • Aedes aegypti
  • Ephemera varia
  • Bellura sp.

Malacostraca

• Defining characteristics:
  • Thorax with eight segments, abdomen with six to seven segments plus a telson
  • Appendages on the sixth abdominal segment are flattened to form uropods
• Body characteristics:
  • 60% of crustaceans; decapods, euphausiids, stomatopods, isopods, amphipods
  • Malacostracan body is tripartite, consisting of a head, thorax, and abdomen
• Representative species:
  • Shrimp-like; large; bottom dwelling; violent carnivores

Learn about the characteristics of Phylum Annelida, including their muscles, setae, and general features. This quiz covers the defining traits of Annelids and their subgroups.