The only Arizona representative is the freshwater hydra Class Hydrozoa, Genus Hydra.

The body is a cylinder, polyp, with five to seven tentacles extending from the mouth. Hydra are typically 1 to 4 centimeters long, ranging in color from light tan to brown. Some hydra are green because of algae living beneath the epidermis.

Reproduction is either sexual or asexual. Hydra reproduce asexually in warm weather by budding. The bud will grow on the adult's body until it becomes large enough to break away. They reproduce sexually in the fall. Some species are hermaphroditic, producing both sperm and egg. Other species have separate sexes. The sperm and eggs are released into the water for fertilization. The fertilized egg forms a capsule to protect the embryo until it hatches in the spring.

The body of the hydra is only two cell layers thick. The epidermis serves for protection. The inner layer, gastrodermis, is the digestive layer. There are two types of cells on the inner layer. Digestive cells that secrete enzymes for digestion, and nutritive cells that absorb the food.

The hydra does not have a central nervous system. Between the cell layers is nerve net. Sensory receptors are located on the tentacles. When touched, these receptors stimulate nematocysts, stinging cells. The nematocysts can shoot a barbed arm out to sting and entangle prey. The tentacles push the prey through the mouth and into the open cavity, or gut. The sting of a hydra is too mild to be dangerous for humans.

Hydra are normally sessile, attached by the basal disk to some object. But they can move from place to place by somersaulting or filling the basal disk with air and floating. Every body of (non-chlorinated) freshwater has a population of Hydra, but without scanning the surfaces of the submerged vegetation with a magnifying glass, you would probably never know. Hydroids are extremely common aquatic animals, but their small size relegates them to obscurity.

• Class Turbellaria - freshwater planarian

  The planarian is a free-living worm from 1 to 3 centimeters long whose body is covered with cilia for swimming. It is a scavenger, usually foraging for carrion along the bottom, but it can catch microscopic animals like protozoa. It feeds by extending a muscular pharynx from it mouth, which is located on the ventral surface. There is only one opening in the digestive system. What food is not digested and absorbed in the intestine is excreted back through the mouth. While the internal structure of the planarian is simple, it is much more complex than the hydra.

  The nervous system of the planarian exhibits cephalization, concentration of nerves in the head area. There are two clusters of cells, ganglia, that form a simple brain. Leading from the brain to the posterior end of the body are two longitudinal nerves connected by transverse nerves. Planarians are sometimes called "cross-eyed worms" because of the two closely spaced eye spots on the head. These are composed of photosensitive cells that help the animal find shaded areas to find food.

  Reproduction is either sexual or asexual. Planarians are hermaphroditic, but do not fertilize their own eggs. Fertilized eggs are attached to rocks or debris and hatch in 2 or 3 weeks. During the summer, planarians reproduce asexually by catching their posterior end on a sharp object and stretching until the body tears in two. Each half then regenerates its missing part. Planarians are very interesting to biologists because of this power of regeration.

• Class Trematoda - parasitic flukes

  Flukes are adapted to parasitism. Most are about one centimeter in length, their body covered with a touch, unciliated cuticle. Some flukes are endoparasites, living inside their hosts while others are ectoparasites, living on the external surfaces of their hosts.

  The sheep liver fluke, Fasciola hepatica, is of economic importance to sheep farmers.

  The sheep liver fluke life cycle:

  • Begin with adult flukes in the liver or gallbladder of sheep.
  • The hermaphroditic adults mate and produce tens of thousands of eggs.
  • The eggs enter the intestine of the sheep and are eliminated in the feces.
  • The eggs hatch in water and the larvae invade snails.
  • Inside the snails, the larvae multiply asexually.
  • The larvae leave the snail and form cysts, dormant larvae surrounded by a protective coating, on blades of grass.
  • When a sheep eats the grass, the cysts hatch in its digestive system.
  • The fluke bores through the sheep intestine into the blood where it matures and completes the cycle.

• Class Cestoda - parasitic tapeworms

  All tapeworms are parasitic, without sense organs, mouths, or digestive tracts. Since they live in the intestine of their host, they can absorb nutrients directly through their body.

  The tapeworm consists of a scolex, head and proglottids, body sections.
  The oldest proglottids are at the posterior end of the worm. Each proglottid contains reproductive organs. Tapeworm eggs can be fertilized by either cross-fertilization or self-fertilization.

  The beef tapeworm, Taenia saginatum, is one species able to infect humans. Human waste disposal and meat inspections today work against the life cycle of the beef tapeworm.

  The beef tapeworm lifecycle:

  • Cattle eat grass contaminated with proglottids and eggs.
  • Larvae hatch and bore through the cow's intestine into the blood.
  • The larvae then burrow into muscle tissue and form cysts.
  • Humans become infected when they eat meat that has not been cooked sufficiently to kill the bladder worms inside the cysts.
  • Once the cysts are inside the human intestine, the cyst wall dissolves releasing the bladder worm.
  • This worm attaches itself to the intesting wall and grows to maturity.
  • The sperm and eggs inside each proglottid producing fertilized eggs.
  • Proglottids break off and are eliminated with the host's feces, completing the cycle.

There are tens of thousands of species of roundworms.
Most species have separate sexes and are free-living.

The only parasitic species important in Arizona infect animals, except for the genus Trichinella, which causes trichinosis from eating undercooked pork. Modern farming practices and meat inspections have made this disease rare in the United States.

The greatest importance of roundworms in Arizona is as food for other animals within aquatic environments. They are common in rich soil and serve as decomposers.

While most of our roundworms are small, from a few millimeters to a few centimeters in length, some are much larger. The horse-hair worm is sometimes found in puddles after a rain. This roundworm can be several inches long, although its diameter is still very small.

Along with sheep, what other animal is part of the life cycle of the sheep liver fluke?

Research Links:

• Pond Life Identification - Microscope Magazine
• Guide To Freshwater Invertebrates - Local
• Invertebrate Animals - Kimball's Biology Pages
• Phylum Cnidaria - University of Michigan
• Phylum Platyhelminthes - University of Michigan
• Planarians Homepage - Planarians.org
• USDA Nematology Lab - U.S. Department of Agriculture
• Nematology 110 - University of California, Davis
• Flatworms and Roundworms - DVHS Biology Class

The freshwater hydra, a member of the Phylum Cnidaria, has the ability to sting.
The sting will immobilize the small animals hydra use for food, but most humans do not even feel it.

In addition to sheep, snails are also part of the life cycle of a sheep liver fluke.