BIOLOGY 03050, INVERTEBRATE ZOOLOGY, WEEK 6: PHYLUM PLATYHELMINTHES
Text (7th ed.): Ch. 19, pp. 416-427, 430-432; Ch. 14, pp. 308-313, 318-326
10/5/98 Dr. W. Crone (303 FTZ, 270-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
possible web site: http://www.who.int/ctd/html/schistodis.html (schistosomiasis)
possible HVCC a/v VT 0440:"Conquest of the Parasites" (to be shown in lecture next week)
1. Phylum Platyhelminthes ("flat worms"): A phylum of 12,000+ species consisting of both free-living and parasitic worms. Notable features are bilateral symmetry, dorsal-ventral flattened bodies, a digestive tract with one opening, and no body cavity (acoelomate status). A bilaterally symmetrical animal is more likely to be active and with a head (cephalization). Many zoologists believe that the acoelomate body plan is the basis for the other invertebrate phyla.
2. Class Turbellaria ("turbulence") (300 species): The free-living flatworms or planarians are aquatic bottom dwellers, both scavengers and predators. Flatworms move by cilia (on a path of mucus made by ventral gland cells) and muscular undulations. A single-opening gut is highly branched to maximize surface area and minimize distance for diffusion. The mouth is ventral, about halfway down, with a muscular pharynx to protrude into prey, soften up food with digestive enzymes, and tear it apart with sucking movements. Space between the muscles and the gut is filled in with mesodermally-derived parenchyma cells--there is no coelom (body cavity). A nervous system with a primitive brain, two eyespots, and auricles (sensory lobes) with sensory cells add to the sensory input at the anterior end. The planarian is flat and hence with a lot of body surface--useful without circulatory or respiratory systems. Planarians are notorious for their abilities of regeneration.
3. Class Trematoda ("with holes") (8,000 species): The flukes are parasites. Single opening digestive tracts as with flatworms, but not as branched. Attachment is through an oral sucker and a ventral sucker (acetabulum). The skin of flukes and tapeworms is called a syncytial epithelium or tegument. In other words, the epidermis is not separated into distinct cells, but consists of a layer of absorptive cytoplasm on the outside and the nuclei underneath muscle layers on the inside. Eggs hatch ciliated larvae called miracidia, which find an intermediate host (often snails) and become bag-like sporocysts (and sometimes rediae). From them arise tadpole-like cercariae, which leave snails to seek the next host. Two major human parasites:
3a. Clonorchis sinensis, the Chinese liver fluke. Adult lives in bile ducts of liver. Eggs out the intestine and out with feces. Cercarial stage finds a fish, penetrates its skin, and encysts. Fluke colonizes human with eating of poorly cooked fish.
3b. Schistosoma species, the blood fluke. Live in bloodstream, with male and female continously copulating. Eggs with spines to help passage in either feces or urine. Miracidia hatches and sporocyst stage matures in snail before next larval stage or cercariae, which penetrate human skin and mature in bloodstream.
4. Class Cestoda ("belt") (4,000 species): The tapeworms are well known intestinal parasites of vertebrates. Tapeworms have no mouth or digestive tract, since absorb food directly through skin. Their skin has small finger-like projections (microvilli) to help with digestion. The adult tapeworm consists of a"head" or scolex with hooks and suckers, a"neck" or germinative zone below the scolex, and the"body" or strobila made of segments called proglottids which bud off from the germinative zone. Proglottids are self-contained reproductive factories, either cross-fertilizing with other tapeworms or among themselves (as the tapeworm is folded up against itself). Both male and female parts are present in each. Thin strips of longitudinal and circular muscle are also present in proglottids, as the worm slowly crawls up"against the current."
4a. Major human tapeworms: Taeniarhynchus saginatus, the beef tapeworm. Adults live in human small intestine. Eggs/proglottids out with feces, six-hooked larvae called oncospheres are ingested by cows in infected fields. Larvae encyst into bladder worm (cysticercus) stage, and when eaten, out pops the scolex and an adult matures.
4b. Taenia solium, the pork tapeworm. In many ways, the pork tapeworm can be more deadly to a human host than the beef tapeworm. The larva (bladder worm, cysticercus) is in a sac or cyst with an inverted scolex that lives in the muscle of a pig. After a person eats infected pork, the larva passes through the stomach into the small intestine, everts the scolex, attaches, and starts to bud off proglottids. In the proglottids, the eggs are produced. The larvae start to develop in the eggs. Most of the time, the ripe proglottids are scattered with the feces and are ingested by a pig to complete the life cycle. Some of the time, however, some of the larva at the oncosphere stage may hatch while still inside the intestine. These larvae treat the human host as the intermediate host and so penetrate the intestine, catch a ride in the blood stream to a new location, and encyst.
6. Aspects of animal development; useful terms to know.
a. zygote: a fertilized egg (union of male and female sex cells)
b. blastomere: cells that are derived from the zygote by cell division
c. blastula: hollow ball formed by the dividing blastomeres
d. gastrula: two layered embryo, often formed from blastula by invagination or gastrulation
e. archenteron: the new cavity in the gastrula formed by the invagination
f. blastopore: the new opening to the archenteron where the invagination occurred
g. ectoderm, endoderm, mesoderm: primary germ layers discussed previously in week 2
The typical developmental pathway of an animal is to have an egg, a zygote, a blastula, a gastrula, a larva, and then an adult. There are two major ways of going about development:
7. Protostome development: typical of most invertebrates. Zygote formation, divisions into blastomeres. These blastomeres tend to be determinate, that is, their developmental fate is fixed from early on. These blastomeres in many protostome groups undergo spiral cleavage to form the blastula. The gastrula is formed by invagination, which creates the germ layers of ectoderm and endoderm, as well as the new opening of the blastopore and the new cavity of the archenteron. The blastopore forms the future mouth and the archenteron forms the future digestive tract. Hence,"protostome," or the first opening (blastopore) being the mouth. Primitive mesoderm cells are created and lie between the endoderm and ectoderm.
8. Deuterostome development: seen in the echinoderms (sea stars) and chordates (phylum that includes vertebrates), thus helping to link them. Zygote formation, divisions into blastomeres. These blastomeres tend to be indeterminate, that is, their developmental fate is not fixed early on. The blastomeres tend to undergo radial cleavage at 90o. Gastrulation occurs, and the now-formed archenteron again will be the future digestive tract. The blastopore becomes the future anus, and the future mouth will develop at the other end.
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Copyright 1998 by Wilson Crone
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