Making Connections. Investigating Invertebrates. Worms: Flat, Round, and Segmented

From 1990-1996 I worked on with the "100 Schools Project." It was California's implementation of the National Science Foundations grant to integrate science content in a new breed of courses. Monte Vista High School was a leader in curriculum development. I was the editor and primary author of a four textbook series. Titled "Making Connections...", it was published from 1996-2001 by the Grossmont Union High School District. The series is now out of print. 

More often than not over the coming months, I'll be reviving this labor of love. I hope you enjoy learning about science in the way I think makes the most sense. Minor editing has been done to the original text. Additions are in highlighted in this color. My titles for the section of the chapter are this color. Most of the diagrams were retrieved in full color from the Internet since the original books were two-colors only. Content in this series is from Volume 4: Making Connections - Integrating the Science of Energy.

Invertebrate organisms have something supporting their bodies besides bones or bony material. They range in complexity from simple multicellular organisms, like sponges, to animals with complex structures and functions, the crustaceans and arthropods.

Many scientists accept a classification system with five kingdoms.

•          Kingdom Monera consists of bacteria and blue-green algae (also known as photosynthetic bacteria).

•          Kingdom Protista includes algae and single-celled organisms (protozoa).

•          Kingdom Fungi includes only the fungi.

•          Kingdom Plantae consists of multicellular land and water plants.

•          Kingdom Animalia is made up of all animals, both with and without vertebrae. Every living thing is classified in one of these kingdoms.

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Week #2

Three Layers

Poriferans have no distinct cell layers. The cnidarians have two distinct cell layers. The next phylum of invertebrate animals we will study has three cell layers. These animals, in Phylum Platyhelminthes, are the flatworms.

A Flatworm. In this cross-section through a flatworm’s body, the three cell layers are clearly visible. The gastrovascular cavity is where digestion occurs. A flatworm has only one body opening, the pharynx.

The three cell layers in the body plan of the flatworms include the endoderm and ectoderm-like cnidarians. The "new" third layer of cells, the mesoderm, is located in the middle, between the endoderm and ectoderm. Ectoderm forms skin and nervous tissue. Endoderm forms digestive tissue. The mesoderm forms most of the other organs in an organism. In fact, when you were in the earliest stages of development, you had three layers of cells, just like the Platyhelminthes.

Platyhelminthes are flattened dorsally. While they are not perfectly flat, they should remind you of a cartoon character who gets run over by steamrollers during Saturday morning cartoons.

Most flatworms are parasites. They feed off a host organism. Tapeworms and flukes are the leading examples of parasitic flatworms. Tapeworms live in the intestines of the host organism. They lack a digestive system of their own—the host digests their food for them. All the tapeworm has to do is absorb pre-digested nutrients. Tapeworms grow very long in their hosts. Three tapeworms removed from a single individual measured nearly 30 meters in total length.

The next most famous flatworm is not a parasite. Planaria are arrow-shaped worms about 3/4 of an inch long. They are free-living organisms that live in freshwater ponds and streams. Planaria have a muscular pharynx which they extend from their digestive systems to suck the juices out of food.

Many experiments have been done using planaria as subjects. Most of these experiments have involved regeneration since the worm can grow new body parts. With a razor blade, an ice cube, and several changes of fresh water, it is possible to grow a worm with two heads (See Figure 42.). After cutting the worm's one original head lengthwise, a two-headed planaria forms in about a week

Just Round It Off

Phylum Nematoda includes roundworms. Nematodes are very similar in structure to the Platyhelminthes, except that they are not flattened. Nematodes are round.

While many nematodes are parasitic, large numbers of free-living worms exist in this phylum. Each cubic meter of garden soil contains billions of these small, round creatures. Of course, too many worms in the soil can damage the roots of plants growing there.

In short, if all the matter in the universe except the nematodes were swept away, our world would still be dimly recognizable, and if, as disembodied spirits, we could then investigate it, we should find its mountains, hills, vales, rivers, lakes, and oceans represented by a film of nematodes. The location of towns would be decipherable, since for every massing of human beings there would be a corresponding massing of certain nematodes. Trees would still stand in ghostly rows representing our streets and highways. The location of the various plants and animals would still be decipherable, and, had we sufficient knowledge, in many cases even their species could be determined by an examination of their erstwhile nematode parasites.

Nathan Augustus Cobb, a nematologist working for the U.S. Department of Agriculture,  1915

Segments Are Something

The final phylum of worms is Annelida. Annelids are worms whose bodies are divided into repeating segments. Earthworms belong to the phylum Annelida.

Earthworms have tiny bristles, seta, on each segment. See Figure 2-29 above. The seta point toward the rear of the organism. They are used in locomotion. As each segment of the worm slides forward, the seta catch on pieces of gravel or dirt.  Acting like small anchors, the seta jam into the dirt and the worm pulls the rest of its body forward. If you have ever tried to pull a worm out of the ground by its tail-end, you know how effective the seta are as anchors.

Earthworms have a circulatory system. The blood travels through blood vessels as it carries food and oxygen to the worm's body tissues. Blood is pumped through the body of the worm by a series of muscular blood vessels known as aortic arches. While the aortic arches do not have the structure of a heart, they function in the same way. Blood enters one end of the aortic arch and is pushed out the other end.

Earthworms react to a variety of stimuli. They avoid light, dryness, and acid. They prefer dark, damp places. The unsophisticated nervous system of the earthworm is not overly spectacular. However, compared to no nervous system (like the sponges), a primitive nerve network (like jellyfish), and the ladder-like nervous systems (like flatworms), annelids are pretty coordinated.

Earthworms eat the dirt they crawl through. Actually, the worms ingest the dirt only at certain times. They can sense something like decaying leaves in dirt they are crawling through. When they sense food, the worm opens its mouth. A large muscular pharynx sucks the food. The material travels down the esophagus and into a storage sack called the crop. Stored material moves from the crop to the muscular gizzard where small pieces of grit and gravel in the dirt/food rub against each other, grinding the food into a pulp. The grinding action of the gizzard is necessary because earthworms have no teeth to crush food.

After grinding, the food passes into the intestine where it is broken down and absorbed. Material not absorbed in the intestine passes out of the body through the anus. This waste material of earthworms is known as castings. Castings help fertilize the soil. The holes in the dirt left by the burrowing worms help aerate soil through which earthworms crawl.

Giant earthworms! Imagine the fish you might catch with one of these!

Leeches are another annelid group. Best known as pests in lakes and ponds, leeches have been used medicinally for centuries. Before the disease process was understood, bleeding—attaching leeches to the body to suck out bad blood—was a common treatment. Since bleeding weakened patients, it was marginally effective at best.

There is now a complete leech therapy branch of medical science. One of my favorite medicinal uses is in ear reattachment and to repair damage in cauliflower ears. Since ears are mostly cartilage, blood flow is low. Since an ample blood supply is a key to healing, leeches are attached to the area close to the stiches. Hirudin is the blood-thinning agent in leech saliva. It keeps blood from clotting in the normal fashion. This is good for the leech and good for the ear reattachment patient, since blood flow does not decrease. 

Medicinal leeches attached to a damaged ear.

Next Making Connections. Investigating Invertebrates: Clams and Sea Stars.

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