Nematodes Overview and Classifications

Phylum Nematoda Overview

• Nematodes, commonly known as roundworms, comprise about 16,000 described species.
• They play various roles in ecosystems, including free-living and parasitic forms.

Class Secernentea (Phasmidea)

• Characterized by paired sensory structures known as phasmids located in the tail region.
• Amphids, similar but less developed structures, are found in the anterior end.
• Notable species include Ascaris, Enterobius, Rhabditis, Tubatrix, Necator, and Wuchereria.
• Approximately 5,000 species categorized under this class.

Class Adenophorea (Aphasmidia)

• Phasmids are absent in this class.
• Most species are free-living; however, some are parasitic.
• Key genera include Dioctophyme, Trichinella, and Trichuris, with around 3,000 species described.

Key Characteristics of Nematodes

• Triploblastic, bilateral symmetry, and vermiform in shape; unsegmented and pseudocoelomate.
• Body structure: round cross-section and covered by a layered elastic cuticle, with molting typical during juvenile growth.
• Features a complete digestive system with a mouth often surrounded by sensory-laden lips.
• Unique excretory systems typically involve one or two renette cells or collecting tubules.
• Body wall contains only longitudinal muscles, enabling specific locomotion.

Terminology and Concepts

• Aschelminth: Refers to phyla divided into those that do not molt (Lophotrochozoa) and those that do molt (Ecdysozoa), including Nematoda.
• Pseudocoelomate: Animals with a pseudocoelom, exemplified by aschelminths.
• Cuticle: A protective organic layer secreted by the hypodermis of many invertebrates.
• Molting: The shedding of exoskeleton or body covering, also known as ecdysis.
• Ecdysis: The process of shedding the exoskeleton in arthropods or the cuticle in aschelminths for growth.

Sensory Structures

• Amphid: Chemosensory organs located at the anterior end of certain nematodes.
• Phasmid: Sensory pits near the tail end of nematodes in the Secernentea class.

Disease Associations

• Trichinosis: An infection from Trichinella spiralis larvae, often from undercooked meat, leading to symptoms like muscular pain and fever.
• Elephantiasis: A chronic filarial disease caused by Wuchereria spp., affecting lymphatic vessels, mainly in tropical locations.
• Heartworm Disease: Parasitic infection in dogs due to Dirofilaria immitis nematode.

Other Related Terms

• Horsehair Worms: Pseudocoelomate animals from the phylum Nematomorpha, also known as Gordian worms.
• Zonite: The individual body unit of Kinorhyncha members.
• Scalid: Complex spines found on certain marine worms, serving various functions.
• Placid: Plates present on Kinorhyncha.
• Introvert: The retractable anterior section of certain worm types, capable of being introverted into the trunk.