EUMETAZOA: PROTOSTOMIA:

In today's lab we will examine two Phyla of Kingdom Animalia that are members of Eumetazoa and characterized by protostome development. The term protostome means "first mouth" which is a reference to the first opening of the digestive system. During gastrulation, the blastopore develops into the mouth while the anus develops later. There are other developmental traits that link protosomes. These will be covered in lecture.

Platyhelminthes and Nematodes are the first phyla we have examined that have all three primary germ layers during development. The three layers are Ectoderm (outermost layer), Mesoderm (middle layer), and Endoderm (inner most layer - digestive tract). One of the traits that differentiate these two Phyla is the presence of a body cavity. Members of Phylum Platyhelminthes have no body cavity and therefore are acoelomate. Phylum Nematoda have a body cavity that is lined by endodermis and mesodermis and therefore are pseudocoelomate.

Phylum: Platyhelminthes- [flat worms] planarians, flukes, tapeworms. (Hickman et al. chapter 14)

Unlike Cnidarians, members of platyhelminthes have bilateral symmetry and varying degrees of cephalization (development of a head). The nervous system is termed "ladder-like" because there are two longitudinal nerve cords with frequent connections between them. Planarians are the Platyhelminthians with the greatest degree of cephalization and many sensory organs are located on the anterior (head) end of the animal. Planarians have eye spots to detect light and auricles for detection of chemicals. Planarians move through their environment head first and therefore can use the eye spots and auricles to gather information about the environment they are entering. They move by either using cilia on the underside of the body or by contraction of muscles, much in the same way earthworms move. Platyhelminthians are acoelomate so they do not have a body cavity but they do have a digestive cavity. Do not confuse the gastrovascular cavity with a body cavity. The digestive system is incomplete so there is a mouth but no anus. As in the Cnidarians seen last week, food enters and leaves the digestive system through the mouth.

Platyhelminthians are the first group we will see with an excretory system to remove nitrogenous waste (ammonia) and to control water balance (osmoregulate). The protonephridia of platyhelminthes is a system of tubes and flame cells throughout the body (Hickman et al. fig 14-6), The flame cells are chambers lined with flagella that drive water into the tubes and eventually out of the body, carrying with it nitrogenous waste.

Many platyhelminthians are parasitic and have complex life-cycles that involve multiple body forms and hosts. Most reproduce sexually and are monoecious, so each individual can produce both egg and sperm (a trait that is adaptive if you are the only parasite in a particular host). Many can also reproduce asexually through fragmentation. Planaria are well-known for their ability to regenerate the missing body parts when they are cut in half.

The most outstanding advances of the group include Bilateral Symmetry (an associated developmetn of the head) and the third germ layer, mesoderm.

Classes of Platyhelminthes:

Tubellaria - free-living flatworms. Planaria best example, move along a slime trail; amazing powers of regeneration.

Trematoda - flukes, parasites, complex life cycles including invertebrates (snails), and vertebrates (fish and humans) = digenea. Construction of Aswan Dam in the Nile Valley of Egypt has lead to the massive distribution of snails in the irrigation canals. A tremendous outbreak (60% of the population) of schistosomiasis followed because the snails are an intermediate host for the human blood fluke.

Cestoda - tapeworms, parasites, have no digestive organs. They live in the digestive tract of other organisms and absorb nutrients that have been digested by the host. The body is long and slender, mostly composed of reproductive organs.

Monogenea - mostly external parasites on fish. Simple life cycle.

Phylum: Nematoda- [thread worms] roundworms. (Hickman et al. chapter 15)

Like Platyhelminthes, Nematodes are protostomes but they differ in having a body cavity. Nematodes are pseudocoelomate, so the cavity (called the pseudocoel) is lined with both endodermis and mesodermis. During the dissection you will open the animals pseudocoel and be able to see mesodermal tissue that lines the body wall and endodermal tissue that composes the digestive tract. The complete digestive tract runs from the mouth on the head to the anus on the animals posterior end. Complete digestive systems have the advantage that the individual can constantly feed and sections can be specialized for processing food in different ways. Although nematodes have a complete digestive system, there is little specialization.

Nematodes have an acellular covering called the cuticle. The cuticle provides protection for the animal, reduces water loss, and is a firm covering to withstand the hydrostatic pressure of the pseudocoelom. Nematodes have a hydrostatic skeleton and move by contracting muscles on alternating sides of the body to wiggle through the environment. Nematodes are the first group we have examined that had been relatively successful in adapting to terrestrial habitats. Although many species are parasitic on plants and animals, nematodes are extremely abundant in soil. While the cuticle reduces water loss, It also stops waste products such as CO₂ and Ammonia from diffusing out of the body. The excretory system is composed of two lateral lines that run longitudinally down either side of the body and removes wastes and maintains water balance. The nervous system of the nematode is composed of a dorsal and ventral never cord.

Nematodes are dioecious and exhibit sexual dimorphism; which means that males and females are morphologically different. Females are slightly larger and males have a "hooked" posterior end and a pair of copulatory spicules used in aligning with the female during copulation.

The second most diverse group of animals, it is estimated there are over 500,00 different species of nematodes. They live in all possible moist and aquatic habitats and represent the most abundant group of animals known (they can number in the billions/ acre of topsoil). Ten species are dangerous to humans as parasites, 50 others parasitize humans with minimum impact.

Ascaris lumbricoides - infect the intestines of humans.
Trichinella spiralis - pigs and humans. Infect muscles.
Necator americanus - hookworm, infect skin from soil.
Pinworms - most common of all human roundworms.

Ascaris Dissection