THE PSEUDOCOELOMATES

We have examined the flatworms, which represent one of many invertebrates that have a general veriform or wormlike shape. Bilateral symmetry and locomotion have resulted in a variety of evolutionary "experiments" of body design.

1) The flatworms are one type of body design and as you have seen,

are small in size and flattened due to limitations of diffusion rates.

Their blind sac limits specialization of the digestive system.

They have simple excretory structures, but no circulatory or respiratory system

Their reproductive systems have been modified to accommodate a parasitic lifestyle.

2) The nemerteans, which I’ll talk about later

3) The coelomates, -I’ll also talk about later,

and 4) The pseudocoelomates that have a high diversity of basic body plans within them.-I’d like to focus on those animals that

The Pseudocoelom

I’ll be talking about some loosely grouped phyla that share little in common, but all possess a pseudocoel

This group includes

1) Rotifers (Rotifera)

2) Nematodes (Nematoda)

3) Nematomorpha

4)Gastrotriches (Gastrotricha)

5) Acanthocephala

6) Loricifera

7) Priapulida

8) Kinorhyncha

It is almost certain that not all of these animals are closely related to each other.

The one thing that they have in common is a pseudocoel-a fluid filled space.

All of these animals were once grouped under the phylum Aschelminthes and used the presence of a pseudocoel to justify the phylum grouping. The pseudocoel probably evolved multiple times, therefore the grouping is very likely to be polyphyletic and thus artificial.

Some of the animals are so small that they have functionally lost their pseudocoel.

The cavity is not completely lined with mesodermal tissue. Only the outside is lined, the inside is endoderm.

A true coelom is completely lined with mesoderm.

The pseudocoel has several functions and benefits

SHOW OVERHEAD OF PSEUDOCOEL IN CROSS SECTION

1)transport system for hormones, nitrogenous wastes, circulatory function

If you think of our own circulatory system, it is a series of fluid filled tubes with a pump attached for transporting hormones, nitrogenous wastes, nutrients, and oxygen. Imagine that tube simplified into a single tube surrounding our digestive system and missing a pump-and wa-la. a pseudocoel

2)hydrostatic skeleton (surface muscles can push against)

strange that animals can have a supportive skeleton made out of fluid

3)suspension of organs (allows for change in size-egg production etc.)-easier for locomotion (organs are bent and squished around in a mesenchymal mass like in flatworms).

4)acellular space-requires no metabolic maintenance

What is a hydrostatic skeleton? How does it work?

The body of animals with pseudocoeloms, especially worm-shaped ones like nematodes-you notice this when cutting into Ascaris-fluid may squirt out.

The fluid inside is pressurized and functions as an antagonistic force against which muscles can act.

SHOW OVERHEAD (FIGURE 9.1-9.3 PECHENIK, P160)

Our own muscles work for really two primary reasons

1) They have a solid material to leverage against, our bones

2) They have another muscle that does the opposite of whatever that muscle did, to return the appendage or body part to its original form

In a worm, circular muscles and longitudinal muscles are antagonistic

Explain worm movement.

That is all well and good for an animal with longitudinal and circular muscles, but some inverts, like nematodes, don’t have any circular muscles. What do they do?

Well there are several requirements for a fluid-filled space to work as a potential antagonistic force against a longitudinal muscle:

Requirements of a hydrostatic skeleton

1) the presence of a cavity housing an incompressible fluid that transmits pressure changes uniformly in all directions

3) that the volume of fluid in the cavity remain constant

4) that the animal be capable of forming temporary attacments to the substrate, if progressive locomotion is to occur on or within a substrate.

You will see with vinegar eels in lab, notice that they are poor swimmers because they are incapable of the peristaltic types of movement that you saw in flatworms-or alternating contractions of longitudinal and circular muscles.

Nematodes locomote very well many free-living nematodes are small and live interstitially in soil or sand-where they can bump between sandgrains or dirt particles well.

SHOW AVOIDING AN ANEURISM OVERHEAD (p. 98 Barnes)

One of the requirements of a hydrostatic skeleton is a covering that avoids kinks and aneurisms. If you have a long water balloon, and bend it in the middle, one side will kink up. For a living organism with a digestive system where that kink forms, may cause serious problems. As a result you see fibers (usually collagen) wrapped in a helical manner around the cylinder. This is why you see this type of fiber pattern in animals with hydrostatic skeletons.

(Triploblastic-bilateral)

2) Cuticle present-molts 4 times during development

3) Cryptobiosis*

4)Unique sense organs -Amphids

5) No respiratory or circulatory structures, well developed nervous system

6) Longitudinal, but no circular muscles

7) Mouth surrounded by six lips

8) Through gut

9) Unique excretory structures (excretory gland cells (renette)/excretory canal system)

10)Exhibits eutely*

11)Determinate cell fate-cleavage unclear

Given the basic characteristics of a pseudocoelom and hydrostatic skeleton, lets consider one of the phyla of pseudocoelomates that may represent a most generalized condition among pseudocoelomates.

CHARACTERISTICS

1)As is the case with nearly all the animals we’ll be covering, from now to the end of the course, nematodes are triploblastic and bilaterally symmetrical.

They are pseudocoelomates, so have a fluid-filled space with mesoderm (longitudinal muscles) on the outside, and endoderm (digestive system) on the inside.

2) They have a thick cuticle-which allows for tolerance to a wide range of environmental conditions. That is flexible, elastic, and tough-one of the basic requirements of a covering around a hydrostatic skeleton. The cuticle consists of three layers all of which contain collagen. There are usually helically arranged fibers throughout the layers.

Nematodes undergo four molts during development. That is they shed their cuticle to grow larger. The cuticle does not continue to grow with the nematodes-so must be shed peridically. Among parasitic nematodes, different molt stages (or instars) are associated with different lifecycle stages. Often times the old molt is retained among parasitic forms as a sheath and is used when entering the bodies of hosts. It is believed that this may function in some way to overcome the immune system of the host.

4) This tough fiberous cuticle resists dessication, heat, and cold and allows many nematodes to encyst or survive in a cryptobiotic state for extended periods of time.

5) Nematodes have strange unique sense organs called amphids found around the mouth. These amphid sensilla are blind sacs on the cuticle and are most developed in marine nematodes. One amphid is situated on each side of the head.

SHOW OVERHEAD PAGE 355 BRUSCA (AMPHID)

The amphids are believed to serve a chemoreceptive function.

Other sense organs include proprioreceptors

These monitor the position of the body and serve as a feedback mechanism back to the brain-The way you recognize the position of your arms and legs without looking at them is largely accomplished through proprioreceptors.

In addition, some nematodes have caudal sense organs called Phasmids that are posteriorly positioned glands.

Other organs include mechanoreceptor sensilla around the mouth, stretch receptors that regulate movement, and simple ocelli in the head as well.

6) There are no respiratory structures-diffusion is the means of respiration. A long thin wormlike form is one means of accomplishing this feat. If you can’t be flat-be really long. The pseudocoel also aids in diffusion and serves in place of a circulatory system. Some nematodes have forms of hemoglobin within their pseudocoel, so there are adaptations for aiding diffusion-especially for those living in anoxic conditions like lake sediment, swamp soil or your colon. The nervous system is well developed as you might guess from the list of different types of sensory organs present on nematodes.

7) They have longitudinal, but no circular muscles-we covered how that works.

8) Nematodes have a through gut. A mouth and anus. This is a major evolutionary innovation with far-reaching consequences. It certainly increases efficiency. Since the digestive system doesn’t re-encounter predigested food and mix it up with undigested food. Since there is a through-gut the digestive system can be specialized and regionalized.

SHOW OVERHEAD PAGE 353 BRUSCA

There is a buccal cavity (mouth cavity)

stomodeal esophagus or pharynx. A pharynx is another name for a muscularized tube that can mash up food.

Sometimes the pharynx may be subdivided into muscular and glandular regions.

The intestines follow. and a subterminal anus.

In males, there is a cloaca which means it receives both digestive waste products and reproductive products. Cloaca literally means sewer.

SHOW FIGURE 19 BRUSCA OVERHEAD (PAGE 354) or 296 Barnes

9) The excretory system is unique in nematodes. They have apparently evolved independently of any other type of excretory system since they do not resemble protonephridia in any way.

There are greatly elongated rennete cells that form an H and exit into a single excretory pore.

10) Another interesting feature of nematodes that exist in some other pseudocoelomates (rotifers & some gastrotriches) is that they are eutelic. Eutely means fixed cell numbers. That is they have a predetermined number of cells over development and many species have the full number when they exit the egg-with the exception of some of the reproductive structures. Growth is accomplished by each individual cell increasing in size.

Each cell beyond the four cell stage can be traced. They have determinate cell fate.

One area of considerable research interest is the developmental biology of nematodes-especially one-Caenorhabditis elegans.

C. elegans is a small nematode, about 1 millimeter in length as an adult- a rather unassuming animal.

It lives in soil, feeds on rotting vegetable matter, and can be cultured in the laboratory on media.

It takes twelve hours from fertilization to egg hatching.

They are transparent, so all activity can be seen through a microscope.

Its growth patterns are predictable: cell divisions can be followed

1 cell>671cells>959cells.

113 cells die along the way as pre-programmed cell death-leaving a total of 559 at hatching and 959 as an adult.

Caenorhabditis elegans is the one of the most studied nematodes in the world. It serves as a model for developmental biologists. Because of euteley, It is convenient to study. It always has 200 nerve cells, 172 cells in the digestive tract and so on. In other words, each cell has a precise fate and position. Because of this property of nematodes, one can precisely map the position of each cell and understand how each cell is involved with the outcome of the developmental program. There is determinate cell fate. Experimental removal and relocation of different cells help to understand how cells chemically communicate with each other.

Einhard Schierenberg (Gottingen FRG) and John Sulston (Cambridge UK) have mappd cell lineages in C. elegans from 1975-1980.

Combined efforts with Robert Horvitz (MIT) who mapped cell lineages from embryo to adult-that is he plotted ultimately what those dividing cells became in the adult worm.

Thus these studies represented the first complete cell lineage study of a complex metazoan.

The entire genome of this species has nearly been sequenced.

Once the cell lines are mapped, it opens the door to understanding the genetic control of development.

Cells have preprogrammed cell fates, depending on the lineage of the dividing cells-not the regulation of those cells. That is cell fate is determined by ancestry rather than what spatial position it holds in the embryo. This is analogous to determinate and indeterminate cell fate found in protostomes and deuterostomes respectively.

Usually cell fate is determined by a combination of genetic program and cell environnment-but in C. elegans, it is based on total gene control.

Many studies have involved removing the female pronucleus from the egg (the female’s nuclear contribution) before fusion.

and have observed that cells undergo several hundred divisions without control of female genes.

Abruptly however, several hours into cell divisions, development stops right when morphogenesis is about to occur (when the embryo starts looking like a worm).

Other studies have found a single cell (called a P founder cell) gives rise to the entire reproductive system.

Other studies have manipulated and modified the developmental process with microsurgery.

Disrupting development in various ways, inserting markers, dyes etc. to follow the cells.

Other studies have transferred cytoplasm to other cells or reduce cytoplasm volume and looked at developmental effects.

Using these techiques, researchers have found proteins that guide developmental for neurons-or serve as a map for wiring patterns.

This line of research has lead to advances in understanding muscular dystrophy and other neuromuscular diseases.

C. elegans studies have served as a prototype for genome mapping techniques

Functional morphologists have modified collagen synthesis to see how different cuticle properties affect locomotory patterns and efficiency.

All in all, a lot of our understanding of development has been generated from studies on a dirt-sucking worm.

Reproduction

Most nematodes are dieocious.

The male usually has a curved posterior for wrapping around the female’s gonopore. Spicules or similar spikey structures are inserted into the gonopore to hold the female during sperm transfer.

A few nematodes are hermaphroditic however. Interestingly, in these, the eggs and sperm are produced in the same gonad-a ovitestis. The sperm are produced first and eggs second, so technically they are protandric hermaphrodites rather than simultaneous hermaphrodites. But unlike most protandric hermaphrodites, there is no cross fertilization. The sperm are stored until the eggs are produced and then fertilize the eggs.

Ecology

There are at least 12,000 described species of nematodes in the world (probably closer to 15,000 described). Some nematologists estimate that there may be as many as 1 million species total. This fact comes from up to 12 new species discovered from a single cubic meter of Midwestern farm soil. This suggests that most people haven’t looked for new nematodes.

There may be a billion nematodes in an acre of soil.

10,000,000 in a square meter of soil

527,000 in an acre of beach sand

90,000 in a single apple.

They tend to be generalized-that is, considering how many species there are, they tend to look very similar to each other-there is not a large amount of morphological divergence.

Their evolutionary success, seems to come from their generalized body form and small size-sort of the same reasons that roaches and rats are successful. They are usually very small. The Ascaris that you dissect in lab is a giant among nematodes.

They are found in every conceivable habitat-marine, terrestrial, freshwater, ice, soil, and, of course, your colon. -not only are they found within every conceivable ecosystem, but they are found in every conceivable parasitic microhabitat of the human body-as well as many other animals. -you can find them in your hair, eyebrows, eyes, nose, throat, ears, skin (in and on), stomach, small intestines, large intestines, rectum, kidneys, lymph nodes, lungs, heart-you get the idea.

Nematodes are important ecologically.

They are decomposers primarily and often are fungal predators--although at least one species of fungus preys on them.

They are of considerable economic importance.

Root-feeding nematodes in grasslands of the Great Plains consume almost twice the biomass of all the cattle feeding in the same area.

The environments and modes of life of nematodes are too diverse to cover here. I would like to point out that there are many common plant parasitic forms that can cause billions of dollars worth of crop damage in the U.S. alone. -But they are also used as pest control agents since many are specific parasites of certain crop pests like boll weevils.