Pseudocoelomate Animals

Rotifers and Nematodes

General characteristics Bilateral symmetry, unsegmented,

triploblastic. Pseudocoel = body cavity between the

endoderm and the mesoderm. Not completely lined with mesoderm = persistent blastocoel.

No peritoneal lining around gut from which to derive the muscles and blood vessels of the body wall seen in "higher" animals.

Blastula

Gastrula

Pseudocoel

General characteristics Generally small, some microscopic,

some long (meter or more). Specialists in miniaturization.

Wormlike body with no definite head. Enclosed in tough resistant cuticle,

often ringed and developed into spines, scales, plates, bristles, etc. Some molt the cuticle during growth (e.g.

nematodes). Cellular or syncytial epidermis beneath

cuticle. Under epidermis is muscle layer usually of longitudinal fibers.

No circular muscles in round worms (nematodes).

General characteristics (digestive system)

Ccomplete, tube within a tube. Mouth, gut, anus.

Advantages of anus Can specialize regions of digestive

system. No need to throw out the good with

digested food. Muscular and well developed pharynx Digestive tract usually only an

epithelial tube

General characteristics

No circulatory or respiratory organs.

Excretory system of canals and protonephridia in some.

Cloaca that receives excretory, reproductive and digestive products may be present.

General characteristics

N.S. of cerebral ganglia or of a circumenteric nerve ring connected to anterior and posterior nerves.

Reproductive system. ‑ Usually separate sexes, male smaller than female.

Relatively simple system.

General characteristics Cell or nuclear constancy prevails =

eutely. The nematode, Caenorhabditis elegans

has 959 somatic cells including neuronal (302 cells representing 118 different subtypes), epidermal (213 cells), muscle (111 cells), and intestinal (34 cells) types.

This makes them important research animals.

For more information go to: http://homepages.ed.ac.uk/aaa/azizweb/Textmed.html#Anchor2771

General characteristics

Predominantly aquatic animals inhabiting both fresh and salt waters, but many nematodes occupy terrestrial habitats. Free‑living and parasitic.

Body Cavity Advantages Increase size without increasing living

tissues. Don't increase volume of tissues so improves surface/volume relationships.

Frees intestine ‑ frictionless movement independent of body wall motions.

Fluid serves as a hydraulic buffer. Waste depository and elimination.

Body Cavity Advantages Hydrostatic skeleton. Acts as antagonist

to muscles. Prob. most important function

Highly advantageous for body cavity to have an epithelium (peritoneum) which: secretes fluid. absorbs wastes or secretes waste products. aids in defense against foreign substances.

Body Cavity Advantages

Most phyla are coelomate ‑ with a true lined coelom at least in the embryo.

Arthropods have lost the coelom and use blood spaces for same purpose.

Coelom is much reduced in molluscs, too. They also use blood spaces.

Phylum Rotifera Definition: Aquatic, microscopic

pseudocoelomates with the anterior end modified into a ciliary organ, the corona, and the pharynx provided with internal jaws, a mastax. Rotifers have a pair of flame bulb protonephridia.

Features Corona ‑ wheel organ. Rotifers

probably evolved from creeping, ventrally ciliated animals. Original corona = ventral ciliated field around mouth used for creeping and food gathering.

Watch particles swept toward mouth by the corona in the lab.

Reproduction Dioecious. Marked sexual

dimorphism w/ males smaller and markedly degenerate.

Males have a reduced digestive tract and no urinary bladder or anus. They mature immediately with sperm during embryonic development.

No males in some species.

Reproduction (parthenogenesis)

Ability for eggs to develop into adults without being fertilized by sperm.

Get seasonal changes in body shape or proportions = cyclomorphosis.

Reproduction depends on inborn cycles and environment. Cyclomorphosis is problem. How do genes and envir. Cooperate to determine form and function?

Rotifer Life Cycle Alternation between mictic and

amictic eggs. Mixis stimulus occurs in late spring or

early summer at population peak. Can be high pop. densities, dietary

substances or increased photoperiod. Most important factors vary with the

species.

Rotifer Life Cycle

In dessicated state can remain dormant for 3‑4 years

Can survive liquid nitrogen (‑272oC) for 8 hrs. Metabolism continues because can show that they use some oxygen.

Regeneration

Can't. Usually they die after amputation often without healing.

Eutely responsible. Theoretically impossible since nuclei don't divide.

Some reports of young rotifers healing and of limited regeneration. How? Interesting developmental problem.

Phylum Nematoda This is one of the most successful phyla

in terms of numbers of individuals and in numbers of species.

They can be found almost anywhere, from land to sea, from mountains to valleys, in polar regions and in the tropics.

They can be free living and are common parasites of both plants and animals.

Nematodes The nematodes are

pseudocoelomate round worms with a flexible nonliving cuticle, no cilia or flagella and no circular muscles.

Since they have no flagella they also have no protonephridia.

Most are very small (< 5 cm), but some parasites can be more than a meter long.

Characteristics The excretory system consists of one or more

large gland cells opening through an excretory pore, or a canal system without gland cells, or both cells and canals together.

The pseudocoele is used as a hydrostatic skeleton.

Extremely high pressure develops in the pseudocoele. This is important because of the lack of circular muscles. Leads to whip like movement characteristic of round worms.

Characteristics

The outer surface of endemic node is covered by a nonliving cuticle.

The cuticle is secreted by a syncytial hypodermis. It is tough flexible and highly resistant to environmental toxins.

The cuticle is one of the nematode preadaptations for parasitism.

Characteristics

Respiration can be both anaerobic and aerobic.

This is another preadaptation for parasitism.

Importance

Many invertebrates, most vertebrates and many plants are parasitized by nematodes.

They can be very important pathogens of humans and domestic animals.

They can also cause great harm to cultivated crops.

Importance Research on the free living

nematode, Caenorhabditis elegans, began in 1963,and in 1998 it became the first organism whose genome was completely sequenced.

Further study will give us insight into the genetic functioning of more complex organisms.

Importance C. elegans contains genes very similar

to genes involved in human disease. Having the worm as a model makes it

possible to study in detail these genes in ways impossible with human subjects.

For more information go to: http://homepages.ed.ac.uk/aaa/azizweb/Tex

tmed.html#Anchor2771

Importance C. elegans is a nematode and can be

used as a model to investigate the parasitic groups in the phylum.

Because of eutely it has been possible to determine the function of the entire nervous system of this worm.

Discoveries made with this nematode can be applied to other animals and even to humans.