Lecture 11

Class Annalida Overview

Annelids are segmented worms divided mainly into two classes:

• Polychaetes: Also known as bristle worms, these organisms are primarily marine and exhibit diverse adaptations to their environments. They display high morphological variety and are characterized by the presence of bristles (chaetae) that aid in locomotion and protection.

• Clitellata: This class includes leeches and earthworms, which are mainly terrestrial and freshwater species. Clitellates are characterized by the presence of a clitellum, a glandular region involved in reproduction and cocoon formation.

Course Logistics

• Reminder for quizzes (10% of final grade): Must be completed by the end of the trimester.

• Field assignment video deadline: Due at the end of next week.

• Timeframe: 22.5 days left in the trimester.

Annalida Characteristics

General Features

• Ecological Importance: Annelids serve as a vital food source for various organisms and play crucial roles in soil aeration and nutrient cycling, particularly earthworms which enhance soil structure and fertility.

• Segmentation: The metameric segmentation of annelids allows for specialization of body segments, which is significant for their evolutionary adaptations and functions within ecosystems.

Phylogenetic Tree Understanding

• Annelida belongs to the protostome group and exhibits bilateral symmetry, providing an evolutionary advantage in movement and organization compared to other animal phyla.

• Parazoa: Includes sponges, which lack true tissues.

• Eumetazoans: True animals with distinct tissues.

• Bilaterians: Animals exhibiting bilateral symmetry, which includes annelids.

Class Polychaeta (Bristle Worms)

Morphology

• Polychaetes exhibit a highly varied body structure with different degrees of segmentation.

• Most segments possess parapodia, which serve multiple functions including locomotion, reproduction, and respiration.

• Kiti (bristle-like structures) vary by species to aid in movement, anchorage, and protection from predators.

Feeding Strategies

• Various Feeding Types:

  • Filter Feeders: Passively capture food particles suspended in the water.

  • Active Predators: Possess sensory structures and well-developed heads for detecting and hunting prey.

  • Deposit Feeders: Consume organic material found within the substrate, with some species being selective about their dietary choices.

Circulatory System

• Polychaetes have an open circulatory system where blood directly bathes the internal organs, facilitating efficient gas exchange and nutrient distribution.

Reproduction

• Most polychaete species are dioecious (having separate male and female individuals), with reproduction occurring through external fertilization during spawning events.

• Development typically involves the larval stage known as trochophore, which disperses in the water column.

Class Clitellata Overview

Main Groups

• Subclass Hyrudinea: Leeches

• Subclass Oligochaeta: Earthworms and their relatives.

Leeches (Subclass Hyrudinea)

Morphology

• Leeches possess a smooth body that lacks parapodia and often showcases approximately 34 annulations, contributing to their flexibility and movement.

• They are equipped with multiple simple eyes, aiding in detecting light; however, they lack complex image formation capabilities.

Behavior and Ecology

• Leeches exhibit varied feeding modes:

  • Active Predators: Such as stabbers and engulfers that capture prey.

  • Ectoparasites: Bloodsucking leeches that utilize hirudin (an anticoagulant) to feed unnoticed by their hosts.

Reproductive Traits

• Leeches are hermaphroditic, possessing both male and female reproductive organs. They typically engage in copulation and fertilization occurs within the clitellum, leading them to lay eggs within protective cocoons with minimal parental care, except in a few species.

Earthworms (Subclass Oligochaeta)

Morphological Traits

• Earthworms comprise approximately 10,000 species and are primarily terrestrial, playing a critical role in maintaining soil health and structure.

• Their head structure is simple, adapted for underground life, with reduced sensory features.

Digestive System

• Earthworms possess a complete alimentary canal designed for efficient nutrient absorption, consisting of the mouth, pharynx, crop, gizzard, and intestine.

• Their castings significantly enrich soil quality when excreted.

Movement and Locomotion

• They utilize peristalsis, which is facilitated by coordinated contractions of circular and longitudinal muscles.

• Their hydrostatic skeleton, supported by a coelom, allows for efficient movement through soil.

Circulatory System

• Earthworms have a closed circulatory system featuring five hearts and a network of vessels that enable effective transportation of nutrients and waste throughout their body.

Reproductive Methods

• Similar to leeches, earthworms are hermaphroditic but do not self-fertilize. They can store sperm and later deposit this sperm into cocoons for protection during egg development.

Key Takeaways

• Understand the distinct characteristics of polychaetes, leeches, and earthworms.

• Recognize the ecological importance of annelids in nutrient cycling and soil health, which is crucial for agricultural and environmental sustainability.