5. Nematoda and Pseudocoelomates

Overview of Pseudocoelomates

  • Phylum Nematoda and other minor phyla

  • Discussed are various phyla that are part of this classification:

    • Cycloneuralia:

    • Includes five phyla with a secreted cuticle that may have chitin:

      • Kinorhyncha

      • Loricifera

      • Priapulida

      • Nematomorpha

      • Nematoda (main focus)

    • Gnathifera:

    • Evolved from complex-pharynx ancestors with a cuticular jaw:

      • Rotifera

      • Acanthocephala

      • Kamptozoa (= Entoprocta)

Major Groups and Classifications

  • Animal Kingdom Classification:

    • Deuterostomia: includes Chordata, Hemichordata, Echinodermata, and Chaetognatha

    • Protostomia: includes groups like Arthropoda, Tardigrada, Onychophora among others

    • Ecdysozoa: characterized by the process of molting (ecdysis)

Phylum Kinorhyncha

  • Described in 1983, consists of tiny marine animals (200-400 µm)

  • Found in marine gravel (interstitial spaces)

  • Characteristics:

    • Body divided into 13 segments (head, neck, trunk)

    • Body covered with a thick cuticle and spines for locomotion

Phylum Loricifera

  • Known as 'penis worms'

  • About 22 known species, found mostly in colder waters

  • Predacious lifestyle; burrow into mud and extend mouths to catch prey

  • Size is around 0.3 mm, with a protective external casing (lorica)

Phylum Nematomorpha

  • Known as Gordian or ‘horsehair’ worms

  • Approximately 351 species known, possibly 2000 in total

  • Juveniles are parasites of arthropods; adults are free-living in freshwater

Phylum Nematoda

  • Over 25,000 named species, with estimates up to 1 million

  • Most widespread and numerous multicellular animals

    • Found in various environments: sea, minerals, polar, tropical, and extreme conditions

  • Key Characteristics:

    • Body Structure:

    • Spindle-shaped bodies, circular in cross-section

    • Covered by a multi-layered cuticle (lipid, protein, fibers)

    • Exhibits bilateral symmetry and is triploblastic

    • Locomotion:

    • High hydrostatic pressure in the pseudocoelom aids in movement through sinusoidal waves

    • Digestive System:

    • Lacks muscular gut; food is moved along solely by swallowing

Nematode Anatomy and Physiology

  • Body Organization:

    • Fluid-filled pseudocoel supports shape and may assist in locomotion against the muscles

    • Muscles are predominantly longitudinal, without circular muscles to assist in movement

    • Key anatomical features include mouth, anus, pharynx, and a simple reproductive system

    • Males possess copulatory spicules for reproduction

Parasitic Nematodes

  • Examples of significant human parasites include:

    • Ascaris lumbricoides: Adults live in the small intestine, with females producing up to 200,000 eggs daily

    • Dracunculus medinensis: Causes guinea worm disease, ingestion occurs via contaminated water

    • Wuchereria bancrofti: Causes lymphatic filariasis or elephantiasis, transmitted via mosquitoes

Life Cycle of Ascaris lumbricoides

  • Adult roundworms reside in the host's small intestine, where they freely reproduce.

  • Eggs are excreted and can remain viable in the environment.

  • Upon ingestion by a new host, juveniles develop and migrate through various tissues including the lungs, before returning to the intestine to mature.

Summary of Parasitic Impacts

  • Widespread Issues:

    • Global prevalence of nematodes results in substantial health issues; about 25% of the population may be infected by Ascaris, causing malnutrition and intestinal obstruction.

    • Elephantiasis is significant due to the massive swellings it causes, leading to disabilities.

Additional Resources

  • Websites for further information on parasites and nematodes available from sources like Ohio State University and travel health organizations.

Unique Features

  • Characterized by a pseudocoelom, which provides fluid-filled support and space for organ development.

Body Plan and Skeleton

  • Body supported by a fluid-filled pseudocoelom, providing internal support.

  • The skeleton is not rigid (no true skeleton) but consists of hydrostatic pressure from the fluid within the pseudocoel.

  • Organized into segments in certain phyla (e.g., Kinorhyncha).

Food Capture and Digestion

  • Many nematodes are predators or parasitic, obtaining food by ingesting organic matter through a simple digestive system without muscular gut; food processed by passive intake and swallowed.

  • Unique structures, such as a cuticular jaw in some Gnathifera.

Respiration

  • Gas exchange occurs via diffusion across body surfaces, typically through the cuticle, due to the lack of specialized respiratory structures.

Internal Transport

  • Gases and nutrients move through the pseudocoelomic fluid, aiding in distribution to cells throughout the body.

Excretion and Water Balance

  • Excretion handled through gland cells that secrete waste; process of osmoregulation is typically osmoregulatory rather than strictly excretory.

  • Some possess specialized excretory organs (e.g., renette cells in nematodes).

Nervous System

  • Generalized nervous system with nerve cords and rings. Sense organs can be simple, like amphids in nematodes, for chemical detection.

Reproductive System

  • Modes of reproduction vary; nematodes often have separate sexes while some, like Rotifera, can be hermaphroditic.

  • Fertilization can be internal (most nematodes) and may exhibit asexual reproduction.

Development

  • Development often involves direct development or, in certain nematodes, larval stages dispersed through eggs, which can be resistant to adverse conditions.

  • Specific larval types may exist, such as juveniles in parasitic species like Ascaris lumbricoides.

Ecology and Evolution

  • Found in diverse environments including aquatic, terrestrial, and extreme habitats.

  • Adaptations include cuticle for protection and structures for survival in various ecosystems, fitting into Ecdysozoa within the Animal Kingdom classification.

Compare and Contrast

  • Similar groups may include Annelids (segmentation) and Arthropods (exoskeleton). While both possess various adaptations for support, their structural system differs (internal vs. external).

General Principles

  • Important to understand general processes like gas exchange, feeding mechanisms, and reproductive strategies when studying animal adaptations.