Phylum Nematoda: Form, Function, and Classification

Chapter 22: Phylum Nematoda - Form, Function, and Classification

  • Nematodes are among the most abundant animals on Earth.

    • Most are small and non-parasitic.

    • Some species cause diseases that significantly affect humans, animals, and plants.

General Characteristics of Nematodes

  • Body Wall and Hydrostatic Skeleton

    • Composed of a cuticle, an external non-living coating.

    • The cuticle is also associated with a glycocalyx, which contributes to the protective and sensory functions of the organism.

Nervous System

  • Main concentrations of nerve elements are associated with various structures:

    • Cuticle: Provides support and protection.

    • Sensilla: Specialized sensory structures that include:

    • Papillae: Tactile organs found both anteriorly and posteriorly.

    • Amphids: Chemoreceptors located at the anterior end.

    • Phasmids: Sensory receptors located at the posterior end.

Digestive System

  • Generally complete in most nematode species.

    • The cuticle lines the buccal cavity, esophagus, and rectum.

    • Nematodes shed cuticular linings including the external cuticle during molting.

  • Buccal Modifications in Parasitic Nematodes:

    • May possess armaments (teeth) for attachment or feeding.

  • Muscular Pharynx Importance:

    • The esophagus is muscular, functioning to suck food and force it into the intestine.

    • Necessary due to high pressure within the surrounding pseudocoel.

  • Differences in Rectal Anatomy:

    • Female Nematodes:

    • Rectum connects the intestine to the anus.

    • Functions also as a cloaca since it receives sperm.

    • Male Nematodes:

    • Rectum connects solely to the anus.

Secretory-Excretory System

  • Typically consists of tubular systems with two canals in the hypodermis connected by a transverse canal near the anterior end.

  • The transverse canal opens to the exterior via the excretory pore.

  • Main Excretion Mechanism:

    • Most excretion occurs through the intestine.

  • Secretory Functions of the System:

    • Enzymes are secreted for the digestion of the old cuticle.

    • Antigenic substances are secreted for the glycocalyx.

  • Osmoregulation Variances by Habitat:

    • Aquatic nematodes must continuously pump out water due to osmotic pressure, unlike those living in more stable or less aqueous environments.

Reproduction

  • Most nematodes are dioecious; few are monoecious and some reproduce via parthenogenesis.

  • Sexual Dimorphism is usually evident in anatomy and behavior.

  • Male Reproductive System:

    • Copulatory Spicules: Structures that open the female vulva.

    • Gubernaculum: A structure that guides spicules into the vulva.

  • Female Reproductive System:

    • Anatomy: Ovary → Oviduct → Uterus → Vagina → Vulva.

    • Spermatotheca: A structure for storage of sperm; also the site where fertilization occurs and completion of meiosis takes place.

  • Mating Communication Mechanisms:

    • Chemotactic Responses: Use of pheromones for attraction.

    • Thigmotactic Responses: Male caudal papillae detect the vulva, triggering a probing response by spicules.

Development

  • Egg Shell Layers:

    • Lipid Layer: Prevents desiccation and blocks entry of water-soluble chemicals.

    • Chitin Layer: Provides physical protection to the developing embryo.

  • Embryogenesis Timing:

    • Eggs embryonate after leaving the female.

    • Females typically lay embryonated eggs and may be ovoviviparous.

  • Hatching Mechanisms:

    • Larvae produce enzymes to degrade the egg shell.

    • For operculated eggs, the larva utilizes a stylet to penetrate the operculum for hatching.

  • Growth and Ecdysis Pattern:

    • Undergo 4 larval stages (L1-L4 or J1-J4) with ecdysis occurring between each stage.

  • Hypobiosis:

    • A state of arrested development in response to adverse environmental conditions, assisting in survival until favorable conditions return.