Slide 1 The next group of bilateria are the pseudocoelomates. Our representative group will be the nematodes (notice that we will not discuss the rotifers).
Slide 2 The pseudocoelomates are those animals with a body cavity that is surrounded by both endoderm and mesoderm (as opposed to the true coelomates that have a body cavity completely surrounded by medoderm).
Slide 3 Phylum Nematoda is a diverse group of worm-like organisms that live in just about every environment on earth. They are much more independent of water than the acoelomate flatworms. They are able to move through their environment through the use of a hydrostatic skeleton - a fluid filled space that provides structural support to the animal. This is also the first animal we are looking at that has a complete digestive tract. This means that there are two openings - both a mouth and an anus. There are both free living and parasitic species. Many of them are beneficial soil species that improve soil fertility and aid plant growth. Other species are nasty parasites that cause disease in a wide variety of organisms.
Slide 4 A hydrostatic skeleton is the skeletal system that is used by the majority of invertebrate animals for movement and support. A hydrostatic skeleton is one that contains no rigid, hard structures or bones for support but rather rely on a fluid-filled cavity usually surrounded by muscles. Hydrostatic skeletons are found in molluscs like this snail, the cnidarians, the nematodes, and other soft-bodied animals.
Slide 5 In the nematodes we see a complete digestive tract - this is not found in the cnidarians or the flatworms - these organisms have an incomplete digestive tract. A complete digestive tract is one in which food is ingested at one end of the tract, the mouth, and wastes from digestion are passed out of the tract at the other end, the anus. A complete digestive tract is an evolutionary advance - it allows for specialization along the length of the tract. For example, different digestive enzymes can be used along different parts of the tract for more efficient digestion.
Slide 6 Phylum Nematoda is an incredibly abundant group of organisms. There are about 200,000 described species and there are most likely many more undescribed species. They are second only to the insects in number of species. These are the most abundant animal in terms of number of individuals making up about 80-90% of all animals. For example, if you were to dig up a cubic foot of soil - about the amount that would fit in a shoe box - there will be about 1.5 million nematodes in the soil. They include beneficial soil organisms that digest organic matter and return nutrients to the soil. They also include some serious parasites such as trichinosis that come from eating undercooked pork, and elephantiasis which is an enlargement of the lower extremeties as a result of blockage of the lymph system. They can also cause serious damage to crop plants.