The document summarizes key characteristics of the aschelminths, which are a group of seven phyla of invertebrate animals including rotifers, kinorhynchs, nematodes, nematomorphs, acanthocephalans, loriciferans, and priapulids. Some unifying features are a pseudocoelom body cavity, digestive tract with muscular pharynx, constant cell numbers, protonephridia excretory system, and a cuticle layer. There are two hypotheses for their evolutionary relationships - either they are related based on shared structures, or they are polyphyletic with similarities due to convergent evolution. Each phylum is then briefly described in terms
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)