Phylum Nematoda Overview

Phylum Nematoda

• Unsegmented roundworms covered in a flexible, nonliving cuticle.
• Ecdysis: process of molting the cuticle as the worm grows.
• Sensory Organs: Paired cephalic setae and amphids on the head.
• Protostomes: bilateral and triploblastic.
• Pseudocoelom: internal organs are suspended in a fluid-filled cavity that acts as a hydrostatic skeleton.
• Belong to the Ecdysozoa clade: characterized by molting cuticles as they grow.
• Molting: regulated by the hormone ecdysone.

General Body Plan

• Typically 1-2 mm long, unsegmented worms.
• Cuticle is made of collagen.

Growth Patterns

• Growth occurs between molts, with only 4 molts throughout their lifetime.
• Eutely: growth occurs through cell enlargement after a fixed number of cell divisions.

Physiology

• Gas exchange: occurs through diffusion. No respiratory or circulatory systems.
• Digestive system: complete gut.
• Excretory system: includes gland cells and canals, opens via an excretory pore.
• Nervous system: Well-developed with a ring of nerve tissue and ganglia around the pharynx, as well as dorsal and ventral nerve cords.
• Sensory system: Includes papillae and setae for touch, anterior amphids, and posterior phasmids (likely chemoreceptors).

Reproduction

• Dioecious: separate sexes.
• Internal fertilization: sperm are ameboid (not flagellated).
• Parthenogenesis: rare form of reproduction where fertilization does not occur.
• Juvenile stages: 4 stages, each separated by a molting event.

Representative Nematodes

• Caenorhabditis Elegans:
  • model organism for neuronal development.
  • first organism to have its entire genome sequenced.

Class Merostomata - Horseshoe Crabs

• Defining characteristics: Appendages on opisthosoma are flattened into ‘book gills’, final segment modified into long spike;
• Only 4 living species, remained relatively unchanged for 450 million years, with a marine habitat;
• At risk because their blood, with the blue pigment hemocyanin, is essential for biomedical research;
• Important ecologically;
• Their abdomen resembles books, referred to as "book gills";
• Gas exchange occurs in water, limited in air;
• Predator of clams and worms

Class Arachnida

• Defining characteristics: Varied opisthosoma appendages including book lungs (internal) to spinnerets;
• Mainly terrestrial;
• Examples of arachnids include spiders, scorpions, mites and ticks;
• Spiders: Solitary carnivores, usually only interact to mate;
• Scorpions: Mainly nocturnal, solitary carnivores, cannibalism common;
• Mites and ticks: Parasitism common, may live on or in association with hosts;

Appendages Modifications

• Chelicerae: Present in all, may be hollowed out for venom delivery (mainly in spiders);
• Pedipalps: Modified into claws in scorpions, blind end appendages in other groups;
• 4 pairs of walking legs;

Specialized Abdominal Appendages

• Opisthosoma often separated from prosoma by a narrow stalk (pedicel) in spiders;
• Spiders have abdominal appendages called spinnerets near the anus for silk release;
• Scorpions have modified final abdominal segment into a stinger;

Scorpions

• Cannibalistic, they can kill and eat other scorpions;
• Not immune to their own venom;
• Two methods to capture prey:
  • Brute force (large body, big pedipalps);
  • Venom (small body, small pedipalps).

Hardening the Exoskeleton

• Crustaceans: Add Ca2CO3 into the procuticle;
• All arthropods: Tanning;
• Protein cross-links form spontaneously in the procuticle immediately after it is secreted by epidermal cells;

Ecdysis

• To increase in body size, the entire exoskeleton must be shed at once;
• Arthropods molt many times before reaching adulthood, and some continue to molt after that;
• Old exoskeleton is degraded by enzymes and split open by allowing water or air in to "inflate" it;
• Under neural and hormonal control;
• Y-organ in the head produces ecdysteroids to trigger moulting;
• Prothoracic glands in the thorax also produce ecdysteroids;

Nervous System

• Complex brain and sensory structures in head region;
• Ventral nerve cord with metamerism (ganglia associated with body segments);

Circulation

• Open/closed hybrid system;
• Blood leaves the heart through closed vessels;
• Blood enters the heart via holes directly from the hemocoel (main body cavity);

Visual Systems

• Two levels of complexity:
  • Ocelli (small cup with light sensitive surface);
  • Compound eye (filled with many stacks of lenses and receptor cells organized into units called ommatidia);
• Can detect UV and polarized light;

Reproduction

• Almost exclusively sexual, some asexual, some parthenogenesis;
• Mix of internal and external fertilization;
• Many larval forms;

Subphylum CHELICERATES

• Defining characteristics: Only arthropods with no antennae, body in two sections with no distinct head, chelicerae (first pair of appendages);
• Includes spiders, scorpions, horseshoe crabs and sea spiders;
• Body divided into two regions: prosoma (head + thorax), opisthosoma (abdomen);

Appendages

• NO antennae, no mandible;
• Clawed appendages called chelicerae on either side of the mouth for food ripping and predation;
• Food manipulators with blunt ends called pedipalps;
• Walking legs (4 pairs);

Ancestors of Chelicerates

• Likely some of the first terrestrial animals;

Subphylum Mandibulata

• Includes centipedes and millipedes;
• Mainly detritovores in moist terrestrial environments;
• Some predators (centipedes);

General Features of Mandibulata

• Two tagmata = head and trunk
• Trunk is comprised of elongated thorax and reduced abdomen;

Segments of Mandibulata

• Segments 1-5 = Head:
  • 1 = Antennae;
  • 2 = Lost during development;
  • 3 = Mandibles (jaws);
  • 4 = Maxilla 1 (flappy mouth parts);
  • 5 = Maxilla 2 (stringy mouth parts);
• Segments 6 onwards = Trunk:
  • 6 = Maxillipeds (centipedes);
  • Paired walking legs (multiple pairs, can be 10s to 100s)

Mouth Parts of Mandibulata

• Flappy primary maxillae form the lower lip;
• Mandibles/jaws are inside the mouth;

Respiration of Mandibulata

• Openings to the outside = spiracles;
• Connected to the inside via tracheal system that delivers O2 to the tissues;

Excretion of Mandibulata

• Malpighian tubules: tubules extending from the back of the gut;
• Absorb solutes and wastes from hemolymph;
• Passes solid nitrogenous waste to the gut for excretion;

Class Chilopoda - Centipedes

• One pair of walking legs per segment, spiracles on the top or side of the body;
• All venomous;
• Predatory;

Appendages Modifications of Centipedes

• 1st pair trunk/thoracic appendages (maxillipeds) modified for venom delivery;

Important Predators of Centipedes

• Large centipede in the South Pacific feeds on seabird chicks;
• Important for nutrient cycling to transfer "marine" nutrients to a terrestrial ecosystem;

Class Diplopoda - Millipedes

• Two pairs of walking legs per segment, spiracles on the bottom of the body;
• Produce noxious chemicals as a defense;
• Detritovores;

Appendage Modifications of Millipedes

• Segments of the trunk are fused together in pairs, resulting in two sets of walking legs per trunk segments;

Defense Mechanism of Millipedes

• Unlike centipedes, millipedes can't bite and are slow moving;
• Option 1 = defense coil;
• Option 2 = release noxious chemicals through holes along the side of their bodies (including cyanide);

MYRMECOPHILY

• Some species of millipedes live in close association with ants (symbiotic and commensal);
• Used by the ants to clear nest space in some cases;
• Sometimes this can turn to a toxic relationship, and if ants are too hungry, they can eat them;