The development of a body cavity (coelom) is considered a major evolutionary advantage over those animals which do not possess a body cavity (acoelomate). Body cavities are advantageous for a number of reasons, such as to provide more room for organ development, to provide an increased surface area for diffusion of gases and/or nutrients, and to facilitate locomotion by serving as hydrostatic skeletons.
A body cavity is characteristic of all bilateral animals above the acoelomates. A true coelom is a cavity in which the inner body wall and the visceral organs are lined by mesodermal tissue. A pseudocoeIom, found in animals to be examined in the present exercise, is defined as a body cavity that is lined by mesoderm externally and endoderm internally.
The nematodes are one of several phyla usually discussed together as the pseudocoelomates because of their shared possession of this structure. Except for this one common features, they are a diverse group of animals, only distantly related. Included in this broad group of animals are the Phyla Rotifera, Nematomorpha, Gastrotricha, Kinorhyncha, and others.
You will examine a member of the phylum Nematoda (Ascaris) as a representative pseudocoelomate. An intestinal parasite as an adult, species of Ascaris are found in the pig, horse, and human.
1. External anatomy
Take an Ascaris from the supply, and place it in a dissecting pan. Gloves should be worn, if available. If not, handle organism with forceps. Do not touch with bare skin. You will need to obtain a female specimen for the dissection below, however you should examine a male specimen as well externally. The sexes are easily differentiated. Male worms are smaller, typically have a hook-shaped sideways bend near their posterior end, and may have tiny copulatory spicules protruding slightly from the cloaca.
You should also find and observe the triradiate mouth at the anterior (smaller) end.
2. Internal anatomy
Lay the worm down in the dissecting pan, and place a pin through the anterior and posterior ends. Use a dissecting needle or fine scissors to cut it open full length along the dorsal midline. Pin back the body walls to expose the internal organs. You may either work dry or, if you prefer, flood the pan (and specimen) with about a quarter inch of water.
Draw the dissected specimen. Locate, and label, the following structures:
- pseudocoel (body cavity)
- nonmuscular intestine
- vagina and uteri - the "Y" shaped structure (the short, stem portion of the "Y" is the vagina; the arms of the "Y" are uteri)
- ovaries and oviducts- the mass of coiled tubules connected to the uteri (the
tubules coiled around the arms of the "Y" are the oviducts; the smaller, terminal
portions are the ovaries) -
Obtain a slide of a cross-section through a female Ascaris. Observe this cross-section using a compound microscope. (A diagram of a cross section through a female Ascaris can be viewed here.) Draw the specimen, labeling the pseudocoel, intestine, uteri, oviducts, muscle cells, dorsal nerve cord, ventral nerve cord, and excretory canal.
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