General Characteristics of Pseudocoelomates

Body Plan: Bilateral symmetry, unsegmented, triploblastic, with a complete gut (tube within a tube).
Systems: Lack circulatory and respiratory systems, relying on diffusion for gas exchange.
Reproduction: Dioecious, meaning separate male and female individuals.
Body Cavities: Possess two main cavities: the gut cavity and a pseudocoel, which forms from the blastocoel.
Coelom Absence: Lack a true coelom (peritoneal cavity), as the pseudocoel is not lined by peritoneum.
Body Wall: Composed of epidermis, dermis, and muscles surrounding the pseudocoel.

Evolutionary Advantages of the Pseudocoelom

Movement: Provides increased freedom of movement.
Organ Development: Offers space for the development and differentiation of digestive, reproductive, and excretory systems.
Storage: Serves as a storage area for wastes.
Support: Functions as a hydrostatic skeleton, providing structural support without bones.

Muscles: Only longitudinal muscles are present in the body wall, located beneath the hypodermis.
Cuticle: A non-living outer layer secreted by the hypodermis, composed of multiple layers of collagen. It provides protection and helps contain the high pressure of the hydrostatic skeleton.
Hydrostatic Skeleton: Fluid within the pseudocoel, combined with muscle contraction, generates movement.

Nerve Cords: Possess two main nerve cords, one dorsal and one ventral.
Sensory Organs: Include sensory papillae at the head/tail and amphids, which are chemoreceptors with modified cilia leading into a deep cuticular pit.

Lifestyle: Many nematodes are parasitic or predatory.
Digestive Tract: Food passes from mouth → pharynx → non-muscular intestine → rectum → anus.
Food Intake: Food is sucked into the pharynx by muscle contraction.
Intestine: The intestine is one cell thick, facilitating nutrient diffusion.
Waste Expulsion: Defecation occurs via the anus, with pseudocoelomic pressure aiding waste expulsion.
Energy Metabolism: In anaerobic conditions (low oxygen), nematodes utilize glycolysis and fermentation to metabolize stored sugars.

Sexual Dimorphism: Most species are dioecious, with males typically smaller than females.
Copulation: Males use copulatory spicules to hold the female vulva open during mating.
Fertilization: Internal fertilization occurs, and eggs are stored in the uterus until deposited.

Prevalence: Affects 800 million people worldwide, with higher infection rates in children and males.
Life Cycle: Lays 200,000 eggs/day, passed in feces. Infective juveniles develop in 2 weeks, resistant to environmental stress. Infection occurs via egg ingestion.
Migration: Juveniles hatch in the intestine, burrow through the intestinal wall, travel to the lungs, are coughed up and swallowed, and mature in the intestine.

Characteristics: Measures 9-11 mm with a hook-like curve on the anterior end.
Life Cycle: Eggs pass in feces. Infective juveniles burrow through skin from contaminated soil, travel in blood to the lungs, are coughed up and swallowed, and mature in the intestine.
Pathology: Cuts into intestinal mucosa with large mouth plates, sucks host's blood, secretes anticoagulant, leading to anemia.

Pathology: Causes trichinosis, a potentially lethal disease.
Life Cycle: Adults burrow into intestinal mucosa, producing live juvenile worms. Juveniles penetrate blood vessels, circulate, and encyst in skeletal muscle, altering muscle gene expression to form nurse cells for the parasite.
Transmission: Transferred by consuming poorly cooked meat containing encysted juveniles.

Prevalence: Most common nematode in the US (30% children, 16% adults).
Pathology: Causes intense anal itching and scratching.
Life Cycle: Adults live in the large intestine/cecum. Females migrate to the anal region at night to lay eggs. Eggs become infective within 6 hours at body temperature. Ingestion of eggs leads to infection.

Prevalence: Eight species infect humans, affecting 250 million people.
Life Cycle: Release live young (microfilariae) into blood/lymph. Mosquitoes ingest microfilariae, which develop to infective stage and are transmitted during a bite.
Pathology: Some species live in the lymphatic system, causing inflammation and blockage, leading to elephantiasis.
Dirofilaria immitis: Dog heartworm, a common U.S. filarial worm transmitted by mosquitoes.

Life Cycle: Humans drink unfiltered water containing copepods with L3 larvae. Larvae mature, and the female worm migrates to the skin surface, causing a painful blister. L1 larvae are released into water when the blister contacts water, continuing the cycle.