1. Chapter 11 The Psuedocoelomate Body Plan:
Aschelminthes (Lophotrochozoan and Ecdysozoan Phyla)

2. Evolutionary Perspective
7 Phyla linked as the aschelminths for convenience
Fossil record is meager
2 Ideas of possible relatedness

3. 1st Hypothesis
The phyla are related based on the presence of the following structures: a pseudocoelom, a cuticle, a muscular pharynx and adhesive glands.

4. 2nd Hypothesis
Contends that the various aschelminth phyla are not related to each other; thus, are probably polyphyletic.

5. Conclusions?
The absence of any single unique feature found in all groups strongly suggests independent evolution of each phylum.
The similarities found among these phyla may simply be the result of convergent evolution as these animals adapted to similar environments

6. General Characteristics
1st animals to possess a distinct body cavity
Lack peritoneal linings found in more advanced animals
Organs lie free in cavity
Cavity is called a pseudocoelom or pseudocoel
Thus AKA Psuedocoelomates

7. General Characteristics (cont.)
Pseudocoel often fluid-filled or may contain a gelatinous substance with mesenchyme cells
Helps in circulation
Aids in digestion
Acts as an internal hydrostatic skeleton that functions in locomotion

8. General Digestion
Most aschelminths have a complete tubular digestive tract that extends from an anterior mouth to posterior anus
Exceptions:
Acanthocephalans
Nematomorphs

9. Eutely
A condition in which the number of cells is constant both for the entire animal and for each given organ in all animals of that species
Ex. Caenorhabditis elegans
Somatic cells = 959
Cells in pharynx = 80

10. Shapes Most are microscopic Some can be over a meter in length
Bilaterally symmetrical
Unsegmented
Triploblastic
Cylindrical in cross section

11. Other Stuff Protonephridia present Mostly diecious
Cuticle may bear spines, scales or other forms of ornamentation that protect the animal are useful to taxonomists
Some aschelminths shed their cuticle in a process called molting or ecdysis in order to grow

12. Location, Location, Location
Most aschelminths are freshwater animals; very few marine species
The nematomorphs & acanthocephalans and many nematodes are parasitic
The rest of the aschelminthes are mostly free-living
Some rotifers are colonial

13. Aschelminthes That Do Not Molt (Lophotrochozoan Phyla)
Phylum Rotifera
Phylum Acanthocephala
Some scientists combine them into a single Phylum Syndermata

14. Phylum Rotifera
Pseudocoelomates with jaws, crowns of cilia and complete digestive tracts
Possess a pair of corona, ciliated organs that aid in locomotion and food gathering
Cilia of the corona do not beat in synchrony; instead, each cilium is at a slightly earlier stage in the beat cycle that the cilium next to them
Gives the appearance of spinning wheels
Once called “Wheel animalicules.”

15. Phylum Rotifera (cont.)
0.1 to 3 mm in length
Mostly freshwater
Less than 10% marine
2000 species divided into 3 classes
Body has approx cells
Organs eutelic
Usually solitary, free-swimming animals
Some colonial forms known

16. Characteristics of the Phylum Rotifera
Triploblastic, bilateral, unsegmented, pseudocoelomate
Complete digestive system, regionally specialized
Anterior end often has a ciliated organ called a corona
Posterior end with toes and adhesive glands
Well-developed cuticle
Protonephridia with flame cells
Males generally reduced in number or absent; parthenogenesis common

17. Feeding and Digestion
The coronal cilia create a current that brings food particles to the mouth
The pharynx contains a unique structure called a mastax (jaws)
The mastax is a muscular organ that grinds food

18. Class Seisonidea
A single genus (Seison) of marine rotifers that are commensals of crustaceans; large and elongate body with reduced corona. Only 2 Species.

19. Class Bdelloidea Anterior end retractile and bearing 2 trochal disks
Mastax adapted for grinding
Paired ovaries; cylindrical body
Males absent
About 590 species
Adineta, Philodina, Rotaria

20. Class Monogononta Rotifers with 1 ovary
Mastax not designed for grinding
Produce mictic and amictic eggs
Males appear only sporadically
1400 species
Conochilus, Collotheca, Notommata

21. Phylum Acanthocephala
Akantha = spine or thorn
Kephale = head
“Spiny headed worms”

22. Phylum Acanthocephala (cont.)
Endoparasites in the intestinal tract of vertebrates (especially fishes)
2 hosts per life cycle
Juveniles found in crustaceans and insects
About 1000 species

23. Size Usually less than 40 mm Exception Macracanthoryncus hirudinaceus
Found in pigs
Can be up to 80 cm long
Can hold up to 10 million eggs

24. Looks
Body of an adult is elongate and composed of a short anterior proboscis, a neck region, and a trunk.
Proboscis covered with re-curved spines
Proboscis provides a means of attachment in the host’s intestine
Females larger than males

25. Aschelminths that Molt (Ecdysozoan Phyla)
Phylum Nematoda
Phylum Nematomorpha
Phylum Kinoryncha
Phylum Loricera
Phylum Priapulida

26. Characteristics of Phylum Nematoda
Triploblastic, bilateral, vermiform (resembling a worm in shape; long and slender), unsegmented, pseudocoelomate
Body round in cross section and covered by a layered elastic cuticle; molting usuallu accompanies growth in juveniles
Complete digestive tract, mouth usually surrounded by lips bearing sense organs
Most with unique excretory system comprised of 1 or 2 renette cells or a set of collecting tubules
Body wall has only longitudinal muscles

27. Phylum Nematoda Nematodes or Roundworms 16,000 species 2 Classes
Secernentea
Adenophorea

28. Class Secernentea Formerly known as Phasmidea
Possess paired glandular or sensory structures called phasmids in the tail region
Poorly developed amphids on anterior
Free living and parasitic
5000 species
Ascaris, Enterobius, Rhabditis, Tubatrix, Necator, Wuchereria.

29. Class Adenophorea Formerly known as Aphasmidia Phasmids absent
Most free-living, some parasitic
3000 species
Dioctophyme, Trichinella, Trichuris.

30. External Features
Body is typically slender, elongate, cylindrical and tapered at both ends.
Amphids are anterior depressions in the cuticle that contain modified cilia and function in chemoreception.
Phasmids are near the anus and also function in chemoreception
Paired ocelli (eyes) are present in aquatic nematodes

31. Important Nematode Parasites of Humans
High reproductive potential
Life cycles that increase the likelihood of transmission from one host to another
Enzyme resistant cuticle
Resistant eggs
Encysted larvae

32. Ascaris lumbricoides The Giant Intestinal Roundworms of Humans
800 million people may be infected worldwide
Live in small intestine of humans

33. Ascaris lumbricoides life cycle
Human ingests embryonated eggs
Eggs hatch in intestine
Larvae penetrate intestinal wall and carried via circulation to the lungs
Larvae molt twice in lungs
Migrate up the trachea and are swallowed
Attain sexual maturity in the intestine, mate, and begin egg production

34. Enterobius vermicularis
The Human Pinworm
Most common roundworm parasites in the US
Lives in lower region of large intestine

35. E. vermicularis life cycle
At night, gravid females migrate out of the rectum to the perianal area, where they deposit eggs containing the 1st larval stage and then die
The females and eggs produce an itching sensation
When a person scratches the itch, the hands and the bedclothes become contaminated

36. E. vermicularis life cycle (cont.)
If the hands touch the mouth and the eggs are swallowed, the eggs hatch
The larvae molt 4 times in the small intestine
Larvae leave the small intestine for the large intestine
Adults mate and females begin egg production

37. Necator Americanus The New World Hookworm Found in SE US
Adults live in small intestine
Feed on blood and tissue fluids
Females can produce up 10,000 eggs daily
Passed through feces

38. Necator Americanus life cycle
Humans become infected when the filari-form larva penetrates the skin, usually between the toes
Outside defecation and subsequent walking barefoot through the immediate area maintains the life cycle in humans
The larvae burrows deep into the skin to reach the circulatory system
The rest of the life cycle is similar to that of Ascaris

39. Trichinella spiralis The Porkworm
Live in the mucosa of the small intestine of humans and other carnivores and omnivores
In the intestine, the adult females give birth to young larvae that then enter into the circulatory system and are carried to striated skeletal muscle

40. Trichinella spiralis (cont.)
The young larvae encyst in the skeletal muscle and remain ineffective for many years
The disease this nematode causes is called trichinosis
Another host must ingest infective meat (muscle) to continue the life cycle
Humans often become infected by eating improperly cooked pork products
Larvae encyst in stomach; move to small intestine; molt 4 times and develop into adults

41. Wuchereria spp. The Filarial Worms
250 million human infected worldwide
2 Examples
Wuchereria bancrofti
Wuchereria malayi

42. Wuchereria spp. (cont.)
Elongate, threadlike nematodes live in the lymphatic system, where they block the vessels
Because lymphatic vessels return tissue fluids to circulatory system, when the filarial nematodes block these vessels, fluids and connective tissue tend to accumulate in peripheral tissues
Elephantiasis

43. Elephantiasis
In the lymphatic vessels, filarial nematode adults copulate and produce larvae called microfilariae
The microfilariae are released into the bloodstream of the human host and migrate to the peripheral circulation at night
Mosquito ingests microfilariae
Molt 2 times in mosquito stomach
Injected back into humans

44. Heartworm Disease Dirofilaria immitis A parasite of dogs
Live in the heart and large arteries of the lungs
Prevention with medicine

45. Phylum Nematomorpha 250 species Horsehair worms or Gordian worms
The hairlike nature of these worms is so striking that they were formerly thought to arise spontaneously from the hairs of a horses tail in drinking troughs or other stock-watering places
No distinct head
Body wall has a thick cuticle, a cellular epidermis, longitudinal cords, and a muscle layer of longitudinal fibers
Two separate sexes

46. Phylum Kinorhynca 1 mm in length Bilaterally symmetrical
Exclusively marine, in mud and sand
No arms or cilia
Burrow through the sand with their snouts
150 species

47. Phylum Kinorhynca 13 or 14 definitive units called zonites
Zonite 1 bears the mouth, and oral cone, and spines
Zonite 2, represented by the neck, contains spines called scalids and plates called placids
The trunk consists of the remaining 11 or 12 zonites and terminates wi the anus
Zonite 11 contains protonephridia
Diecious

48. Phylum Loricifera Recently described (1983) Example Species 14 species
Nanaloricus mysticus
14 species

49. Phylum Priapulida 16 species 2mm to 8cm
Marine worms found in cold water
Priapos = Phallus
Priapos the Greek god of reproduction
Separate sexes but not superficially distinguishable