1. Chapter 12
Smaller Ecdysozoans

3. Diversity Many protostomes possess a cuticle
Non-living outer layer secreted by epidermis
Cuticle restricts growth and must be molted via ecdysis
Members of Ecdysozoa molt cuticle as they grow
Regulation of molting achieved by the hormone ecdysone

5. Phylum Nematoda: Roundworms
12,000 species have been named
As many as half a million may exist
Found in virtually all habitats in all biomes
Topsoil may contain billions per acre
Nematode parasites exist in nearly all animal and plant species

6. Phylum Nematoda: Roundworms
Form and Function
Outer body covering is a thick, noncellular cuticle, secreted by the underlying epidermis- hypodermis
Cuticle serves to contain the hydrostatic pressure exerted by fluid in the pseudocoelom
Collagen is the primary protein in layers of the cuticle
Muscles
Longitudinal muscles lie beneath the cuticle
No circular muscles
Run in four bands, marked off by epidermal cords that project inward to pseudocoelom
Unlike other animals, the muscle extends to the nerve cords

8. Phylum Nematoda: Roundworms
Hydrostatic skeleton-muscle contractions press on fluid
No circular muscles to compliment longitudinal muscle in movement (antagonist) so the cuticle assists.
Fluid is force to opposite side when longitudinal muscles constrict forcing the cuticle to expand
Compression and stretching of the cuticle returns the body to resting position when muscles relax
Produces the thrashing movement of nematodes

9. Phylum Nematoda: Roundworms
Digestion
Gut tube consists of mouth, pharynx, non-muscular intestine, short rectum and anus
Muscular pharynx sucks food in
Intestinal wall is one cell thick-no muscles
Food moves back as new food enters and the body moves
Defecation occurs from opening the anus and allowing pseudocoelomic pressure to expel waste
Some parasitic adults have an anaerobic metabolism; aerobic metabolism are absent
Free-living nematodes and free-living stages of parasitic nematodes have both anaerobic and aerobic metabolism

10. Phylum Nematoda: Roundworms
Ring of nerve tissue and ganglia around the pharynx give rise to small nerves to the anterior end and to two nerve cords, one dorsal and one ventral
Sensory organs at head and tail
Most are dioecious with males smaller than females
Male has copulatory spicules
Fertilization is internal
Nematode sperm has no flagella, in female reproductive tract sperm is ameboid and moves by pseudopods
Eggs are stored in uterus until deposited
Cuticle is shed between each of four juvenile stages

11. Figure 12_03

12. Representative Nematode Parasites
Some are parasites of humans
Most are tropical
Ascaris lumbricoides
Occurs in up to 25% of people in some areas of the southeastern U.S.
More than 1.27 billion affected worldwide
A. suum is found in pig intestines
A female Ascaris may lay 200,000 eggs a day, which pass out in host’s feces

13. Figure 12_04a

14. Representative Nematode Parasites
Survive for long periods in soil
When humans eat undercooked vegetables contaminated with shelled juveniles, or when children put soiled fingers or toys in their mouths, consumed juveniles hatch, and burrow through intestinal wall
Carried through the heart to the lungs, they enter into alveoli causing pneumonia and are carried up to tracheae
Coughed up and swallowed, they mature in the intestine
They feed on intestinal contents and may block the intestines

15. Figure 12_04b

16. Representative Nematode Parasites
Hookworms
Anterior end of these small worms has a hook-like curve
Necator americanus, most common hookworm.
Sexes are separate
Large plates in mouth cut into intestinal mucosa and suck host’s blood
Pump through more blood than they digest
Heavy infections cause anemia
Eggs pass out in feces and juveniles hatch in soil
If human skin comes in contact with soil, infective juveniles burrow through skin to blood
Travel in blood to the lungs, are coughed up to be swallowed, and mature in the intestine

17. Figure 12_05

18. Representative Nematode Parasites
Trichina Worm
Trichinella spiralis causes a potentially lethal trichinosis
Adult worms burrow into intestinal mucosa of the host’s small intestines and females directly produce juvenile worms
Juveniles penetrate blood vessels and circulate throughout the body to all tissues and spaces
Penetrate skeletal muscle cells, redirecting gene expression of the musculature
Cells lose striations and becomes a nurse cells to the parasite
When poorly cooked meat (pork or bear) containing encysted juveniles is swallowed, worms are liberated and mature in the intestine

19. Representative Nematode Parasites
Infect humans, pigs, rats, cats and dogs
Pigs can become infected eating uncooked scraps of infected meat or rats
Four other sibling species with variable distribution, freezing resistance, etc.
Heavy infections cause death
25 cases of trichinosis are reported per year in the US

20. Figure 12_06

21. Representative Nematode Parasites
Pinworms
Most common worm parasite in the U.S.
Adults live in large intestine and cecum
Females, about 12 mm in length, migrate to anal region at night and lay eggs, causing itching
Scratching the anal region contaminates hands and bedclothes
Eggs develop rapidly and become infective within six hours at body temperature
When swallowed, hatch in duodenum and mature in large intestine
Members of this order have haploid males from unfertilized eggs
Females are diploid and come from fertilized eggs (haplodiploidy)

23. Representative Nematode Parasites
Filarial Worms
Eight species of filarial nematodes infect humans
Some cause serious diseases
Wucheria bancrofti and Brugia malayi live in lymphatic system
Cause inflammation and obstruction of the lymphatics vessels
Females release live young, tiny microfilariae, into blood and lymph
Mosquitoes ingest microfilariae when they feed
Worms develop to infective stage and move into the mosquito bite wound when it bites a human
Elephantiasis is caused by repeated exposure
Swelling and growth of connective tissue causes enormous swelling of body parts-scrotum, legs, arms

24. Representative Nematode Parasites
River blindness or onchocerciasis is carried by black flies and infects 37 million people in tropics
Dog heartworm, Dirofilaria immitis, is carried by mosquitoes and is the most common U.S. filarial worm
Heartworm pills

25. Figure 12_08

26. Figure 12_09

27. Representative Nematode Parasites
Dracunculus medinensis, the guinea worm
Larvae live within planktonic copepods (water fleas)
Upon ingestion of contaminated water, water fleas are digested by human host, but larvae survive and penetrate the stomach/intestinal wall
Worms live and mate within body cavity
Gravid females migrate through tissues to the lower extremities and produce an open ulcer
Causing burning pain, the human host immerses leg into water and female releases thousands of eggs into water
Eggs are eaten by copepods and the cycle continues

28. Representative Nematode Parasites
Only known treatment is to carefully remove ulcerated females carefully with a stick; treated this way for thousands of years

30. Phylum Nematomorpha Diversity “Horsehair worms” resemble coarse hairs
Adult structures resemble those seen in nematodes: cuticle, epidermal cords, only longitudinal muscles, and a similar nervous system pattern
Currently placed as the sister taxon to nematodes
About 320 species are known
Occur worldwide
Adults are free-living in moist habitats
Juveniles are parasites of arthropods

31. Phylum Nematomorpha Digestive system is vestigial
Larvae encyst within host and do not emerge from an aethropod host unless water is nearby
Juveniles of freshwater forms use terrestrial insects as hosts
Marine nematomorphs infect certain crabs
Digestive system is vestigial
Larvae absorb food from arthropod hosts
Adults can absorb organic molecules through vestigial gut and body wall

32. Figure 12_10

33. Phylum Tardigrada Known as “water bears”
Very small, less than 1 mm long
About 900 live in a water film around mosses and lichens
Some live in freshwater and some are marine
Most are terrestrial that occupy a film of water surrounding mosses and lichens
Trunk bears eight short unjointed legs, each with claws
A pair of stylets and sucking pharynx protrude to pierce nematodes or plant cells
Body covered by non-chitinous cuticle that is molted four or more times during lifetime
Most of the body cavity a hemocoel
No circulatory or respiratory systems

34. Tardigrades can dehydrate from 85% water to only 3% water
Cryptobiosis
Terrestrial tardigrades can suspend metabolism to survive harsh conditions
Tardigrades can dehydrate from 85% water to only 3% water
In this state they can resist extreme temperatures, ionizing radiation, oxygen deficiency, etc. for years
When water is available, they become metabolically active again

35. Reproduction Sexes are separate
In parthenogenetic freshwater and moss-dwelling species, males are unknown

36. Figure 12_15

37. Phylogeny and Adaptive Diversification
Evolutionary relationships among ecdysozoans are not well-understood
Members of this clade do not share a common cleavage pattern
Nematodes and nematomorphs
Cleavage is unique, not spiral or radial
Cleavage in tardigrades
Has yet to be studied

38. Phylogeny and Adaptive Diversification
Recent phylogenies place Nematoda and Nematomorpha as sister taxa since they share a collagenous cuticle

39. Phylogeny and Adaptive Diversification
Tardigrades have some similarities to rotifers, particularly in their reproduction and cryptobiotic tendencies
Tardigrades and arthropods also share arthropod-type setae and muscles inserted on the cuticle

40. Phylogeny and Adaptive Diversification
Nematodes show the most impressive adaptation
Found in almost every habitat available to animals
Body structure is plastic enough to allow adaptation
Life cycle ranges from simple to complex
Have been known to survive in suboptimal conditions