bio 205 chapter 16: the nematodes

phylum nematoda

roundworms
terrestrial, aquatic or parasitic
most abundant multicellular animals
increased musculature with longitudinal and circular muscles
bilateral symmetry
unsegmented, triploblastic pseudocoelomates
one of the first animals to possess a complete digestive gut

cuticle

an acellular protective covering of collagen secreted by the underlying epidermis that must be shed through ecdysis (molting) so the worm can grow in size

pseudocoelom

the internal cavity in which all of the organs sit and separate the cuticle from the internal organs

nervous system

nerve ring near anterior end
dorsal, ventral and (2) lateral nerves extend posteriorly from nerve ring

sensory structures

2 amphids, 4 cephalic sensilla, 6 outer and inner labial sensilla, phasmids

amphids

major chemosensory organs located laterally on the head within pits, “C-shaped”

sensilla

projections off the anterior end that are used for sensory abilities

phasmids

similar to amphids and are located at the posterior ends of some nematodes

buccal cavity with a stoma

wide, feed on bacteria in the soil

buccal cavity with a needle-like stomatostyle

a parasite feeding from plant roots

buccal cavity that has teeth

a carnivore feeding on protozoans

buccal cavity with enlarged jaw-like teeth

an intestinal parasite to animals

male reproductive system

“J-shaped”
testes produce sperm that are stored in seminal vesicles
during mating sperm are moved through sperm duct (vas deferens) and out cloaca into female reproductive tract

cloaca

an opening shared by the reproductive and excretory systems

female reproductive system

“C-shaped”
ovaries produce eggs
sperm collected during mating is stored in seminal receptacle
eggs are fertilized in uterus and moved through vagina to be released

free-living nematodes

most are soil dwelling decomposers
example: Caenorhabditis elegans (C. elegans) → first multicellular organism to have its entire genome sequenced

parasitic nematodes

example: Ascaris lumbricoides (causes ascariasis), Trichinella spiralis (causes trichinosis)
female worms are larger and thicker, male worms are thinner and have a hooked posterior end

ascariasis

an infection of the small intestine blood vessels in humans

Ascaris lumbricoides

1) adult worms living in a human’s small intestine blood vessels reproduce
2) fertilized eggs are released with human’s fecal matter into environment
3) eggs develop in the environment
4) if consumed, they develop into adult worms within the new host

Trichinella spiralis

a parasitic nematodes that causes trichinosis
results from a human consuming infected pork with larval cysts that is undercooked

heartworms

group of parasitic nematodes that can damage the hearts of dogs and cats
typically transmitted by mosquito species
adult worms produce microfilariae larvae that are picked up by a mosquito and transmitted to another potential host

hookworms

filarial nematode parasites
1) adult worms live in the small intestine and reproduce
2) fertilized eggs are released with human’s fecal matter and develop into microfilariae larvae
3) larvae can penetrate skin and migrate to lungs
4) migrate up the trachea, over the epiglottis and down the esophagus to the small intestine to mature into adult worms

pinworm

infections are common in young children and toddlers
adult female worms with fertilized eggs migrate from small intestine to the rectum out the anus at night
collections of sticky eggs are deposited outside anal orifice
a child may scratch the area and re-infect themselves or pass it to another child

Wunchereria bancrofti

causes lymphatic filariasis or elephantiasis
transmitted by mosquitos
adult worms and microfilariae larvae migrate into lymphatic system causing lymph nodes and vessels to retain water → causes legs to swell and look “elephant-like”

other animal hosts

fish, whales, seals, sea lions, walruses, insects