3. Aschelminths (Gr. askos, bladder +helmins, worm)
1. The aschelminths comprise seven phyla:
Rotifera
Kinorhyncha
Nematoda,
Nematomorpha,
Acanthocephala,
Loricifera
Priapulida
2. The major unifying aschelminth features:
• Pseudocoelom: The pseudocoelom is a type of body cavity that develops from the blastocoel (the primitive cavity in the embryo)
and is not fully lined by mesoderm, as in the true coelomates.
• In the pseudocoelomates, the muscles and other structures of the body wall and internal organs are in direct contact with fluid in
the pseudocoelom.
• Digestive tract
• a muscular pharynx
• constant cell numbers (eutely)
• Protonephridia
• a cuticle
• adhesive glands.
4. EVOLUTIONARY PERSPECTIVE
• Two phylogeny hypotheses:
1. The first hypothesis contends that the phyla are related based on the
presence of the following structures: a pseudocoelom, a cuticle, a
muscular pharynx, and adhesive glands.
2. The second hypothesis contends that the various aschelminth phyla are
not related to each other; thus, they are probably polyphyletic.
• The similarities among the living aschelminths may simply be the result of
convergent evolution as these various animals adapted to similar
environments.
• Common ancestor might have been a primitive, ciliated, acoelomate
turbellarian.
5. GENERAL CHARACTERISTICS
• Distinct body cavity.
• Lack mesentary, organs lie free
• Pseudocoelom fluid-filled or contain a gelantinous substance
• Most have complete tubular digestive tract from mouth to anus which allows for
mechanical breakdown of food, digestion, absorption and feces formation
• Most are microscopic (some grow to over a meter)
• Bilaterally symmetrical
• Unsegmented
• Triploblastic
• Cylindrical in cross section
• Most are dioecious (reproductive organs are in separate animals)
6. Cont.…...
• Cuticle present: may bear spines or scales and is useful for protection
and taxonomic identification
• Molting or ecdysis shed their cuticle.
• Eutely – Same number of cells for each animal and for each given
organ
• Ex. Caenorhabditis elegans (a type of nematode) has 959 cells
• Every worm has 80 cells in their pharynx.
• Protonephridia.
• Cephalization
7. PHYLUM ROTIFERA (L. rota, wheel +fera, to
bear)
• Corona (Gr. krowe, crown),– ciliated organ around the head used for locomotion and
food gathering.
• 0.1 to 3 mm in length.
• Most are freshwater (less than 10% marine).
• wheel animalicules ?
• 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
8. EXTERNAL FEATURES
• Epidermally secreted cuticle used for protection
• Lorica – thickened cuticle that makes an encasement used for
protection and support
• Epidermis is syncytial.
• Head has a mouth, brain, sensory organs.
• Foot has 1-2 toes.
• Foot has pedal glands.
10. PHYLUM KINORHYNCHA (Gr. kinein, motion + rhynchos, snout)
• Less than 1 mm long.
• Elongate, bilaterally symmetrical.
• Exclusive to marine environments.
• No external cilia or locomotor appendages.
• Burrow in mud & sand with snouts.
• 150 species.
• Dioecious.
• Feed on diatom & algae and organic matter.
11. EXTERNAL FEATURES
Body surface:
• Lack cilia
• Composed of 13 or 14 definite units called zonites.
• zonite 1: The head: Bears the mouth, an oral cone, and
spines.
• zonite 2: The neck: Contains spines called scalids and
plates called placids.
The head can be retracted into the neck.
• Remaining 11 or 12 zonites: The trunk: Bears a pair of
lateral spines and one dorsal spine in each trunk
zonite. Terminates with the anus.
Body wall:
• Consists of a cuticle
• Epidermis
• Two pairs of muscles: dorsolateral and ventrolateral.
• Pseudocoelom is large and contains amoeboid cells.
12. PHYLUM NEMATODA (Gr. nematos, thread)
• Triploblastic, bilateral, vermiform (resembling a worm in shape; long
and slender), unsegmented, pseudocoelomate.
• Body round in cross section and covered by a layered cuticle; molting
usually accompanies growth in juveniles.
• Complete digestive tract; mouth usually surrounded by lips bearing
sense organs.
• Most with unique excretory system comprised of one or two renette
cells or a set of collecting tubules.
• Body wall has only longitudinal muscles.
13. EXTERNAL FEATURES
• Slender, elongate, cylindrical, and tapered at both
ends.
• Outer, noncellular, collagenous cuticle.
• Cuticle may be smooth, or contain spines, bristles,
papillae (small, nipplelike projections), warts, or
ridges.
• Three primary layers of cuticle:
o cortex,
o matrix layer
o basal layer.
• Cuticle Functions:
o Internal hydrostatic pressure
o Provides mechanical protection, and
o Resists digestion by the host.
The cuticle is usually molted four times during
maturation.
14. Cont.……
• Epidermis or hypodermis: Beneath the cuticle, surrounds the pseudocoelom.
• May be syncytial.
• Longitudinal muscles: Cause locomotion.
• Contraction of these muscles results in undulatory waves that pass from the
anterior to posterior end of the animal, creating thrashing movements.
• lack circular muscles.
• Some have lips, some have spines or teeth on those lips
• Sensory organs:
Amphids – chemoreceptors along the cuticle
Phasmids - chemoreceptors near the anus
Ocelli – eyespots found in aquatic nematodes
15. PHYLUM NEMATOMORPHA
• Horse hair worm / Gordian worm
• Bilaterally symmetrical, and vermiform.
• They are present on body hair of horse
• Body wall has thick cuticle,cellular epidermis,logitudnal cards amd muscle
layer of logitudnal fiber .
• Sexes are separate
• When eggs mature they leave the body and enter into the water, then
growth/Completion of eggs occur in water, and enter in another organisms
through water.
• Next species generate through eggs in water
• Has no circulatory system (no blood system)
