زواحف 102 - المحاضرة 9: أنيليدا

Zoology 102 - Lecture 9: Annelida

• Phylum Annelida: Composed of segmented worms, including Polychaetes, Oligochaetes, Hirudinida, and Archiannelida.
• Class 1: Polychaeta:
  • General Features: Marine worms (carnivorous) with elongated segmented bodies. Head includes prostomium, peristomium, eyes, tentacles, cirri, and palps. They have a spacious coelom with intersegmental septa. Segmentally paired nephridia and separate sexes. External fertilization and trochosphere larva. Clitellum is absent.
  • Order: Errantia: Free-swimming, similar segments, parapodia (paired appendages) with chaetae (bristles) and cirri, head with sensory organs, pharynx with chitinous jaws and teeth. Examples include Nereis and Aphrodite.
  • Order: Sedentaria: Burrowing forms divided into two or more regions with unlike segments. Parapodia are reduced or absent, and head with reduced sensory organs. Examples include Arenicola and Sabella.
• Class 2: Oligochaeta:
  • Characteristic features will be explored if this class is in the lecture content.
• Class 3: Hirudinida:
  • Characteristic features will be explored if this class is in the lecture content.
• Class 4: Archiannelida:
  • Characteristic features will be explored if this class is in the lecture content.

Polychaeta Examples

• Examples of Polychaetes discussed (like Aphrodite, Sabella, and Arenicola) and their taxonomic classifications.

Neanthes (Nereis sp.) General Features

• Shape and size: Long and narrow, bilaterally symmetrical, tapering posteriorly and relatively broad anteriorly.
• Body structure: Dorsoventrally flattened, segmented body divided into similar segments (except the last one).
• Body parts: Head, trunk, and pygidium.
• Parapodia: Structures on each segment for locomotion and respiration.

Annelida Body Structure and Features

• Head: Well-developed, includes prostomium (not a true segment) and peristomium. Sensory organs like eyes, tentacles, and palps present.

• Trunk: Rest of the metameric body.

• Pygidium: End of the body. Not a true segment, but bears anal cirri.

• Parapodia: Paired lateral outgrowths on each segment, except the prostomium and pygidium. Contain dorsal and ventral acicula (supporting structures) and chaetae (bristles). Muscular attachments for locomotion and respiration.

• Chaetae/Chaeta: Stiff, chitinous rods for locomotion within parapodia. Three main shapes are mentioned as typical, long blade, and oar-shaped.

• Digestive System: Foregut includes pharynx with chitinous buccal cavity. Digestion happens within the alimentary canal: with midgut for digestion and absorption; the hindgut for elimination.

• Respiration: Gaseous exchange occurs through parapodia and body wall, with respiratory pigments aiding absorption.

• Blood Vascular System: Closed circulatory system with ventral blood vessel and network of capillaries facilitating nutrient transport.

• Excretory System: Nephridia, ectodermal structures, are metamerically arranged, involved in waste elimination and osmoregulation.

• Nervous System: Well-developed, with a cerebral ganglion (large brain), circumpharyngeal connectives, and ventral nerve cord. A ladder-like structure.

• Reproduction: Separate sexes with temporary gonads during the breeding season, found within the coelomic epithelium. External fertilization.

• Trochosphere Larva: A ciliated, free-swimming larval stage is mentioned for some annelids.

Annelida Classification

• Summary table provided with external segments, chaetae, and number of chaetae for different classes and examples.