BIOLOGY 03050, INVERTEBRATE ZOOLOGY WEEK 5: CNIDARIA AND CTENOPHORA
Dr. W. Crone (303 FTZ, 270-7439, cronewil@hvcc.edu, http://www.hvcc.edu/academ/faculty/crone/index.html)
9/28/98 Text (7th ed.): Ch. 6, pp. 138-141; Ch. 18
possible web sites: http://www.ucmp.berkeley.edu/cnidaria/cnidaria.html
(focus on the above two phyla)
http://www.ucmp.berkeley.edu/phyla/metazoasy.html
(a great resource for this semester, covering the invertebrate phyla)
possible HVCC a/v source: VT 1174"Life on Earth: Building Bodies" (to be seen in recitation)
1. The sponges demonstrated a"cell aggregation" level of formation. A sponge seems to be organized as a colony of cells all performing their roles on an individual basis. The rest of the animals this semester are organized into tissues (and at least some organs) derived from the germ layers of ectoderm (skin and nerves), endoderm (gut); and mesoderm (everything else).
tissue: similar cells organized to perform a common function. Four types in animals:
1) epithelial: covering
2) nervous: communication
3) connective: support
4) muscle: contraction
2. Phylum Cnidaria ("nettle") (or older name Coelenterata ["hollow gut"]): A phylum of 10,000 species consisting of mostly marine, but some freshwater, organisms including hydras, jellyfishes, corals, sea anemones, and the Portuguese man-of-war. Main characteristics: radial symmetry, tentacles with stinging nematocysts, one-opening gut, and two layers of epidermis and gastrodermis with"jelly" (mesoglea) in between.
3. Class Hydrozoa ("hydra animal") (2,700 species): in lab: Hydra, Obelia, and Physalia
3a. Hydra is a polyp. It has a mouth or oral end with a whorl of tentacles on an elevated portion called the hypostome, a tubular body or column, and an attached aboral end with a basal disc. The tentacles contain cells called cnidocytes, which produce nematocysts or stinging structures that are unique to and indicative of the Cnidaria. The fluid-filled capsule will then flip its lid and launch a coil with a barb. These nematocysts penetrate the animal that triggered them and paralyze the animal with neurotoxins. The tentacles then wrap around the animal and draw it into the mouth, where it is swallowed by muscular contractions. Once in the gut, gland cells then secrete enzymes to break down the food and contractions help to move the nutrition around. Reproduction in the hydra takes two forms: 1) asexual budding, 2) sexual (sperm released into the water from one hydra hopefully finds a ripe egg on another hydra to fertilize).
3b. Obelia. The best way to understand the general cnidarian life cycle is to examine a colonial genus like Obelia. This is a hydrozoan that shows both stages of the cnidarian life cycle, the polyp and the medusa. The polyp is the hydra-like form. In contrast, the medusa (jellyfish) form is free swimming, with the mouth surrounded by tentacles (usually) facing downward. In Obelia, both the polyp and the medusa form are present. The polyp is asexual and the medusa is sexual with separate males and females. Starting with the medusae, eggs and sperm are released in the water. They fuse to form a zygote, which will mature into a simple larval form known as a planula. This larva or planula swims, attaches, and forms a polyp. By asexual budding, it starts a colony of polyps with different functions. In the case of Obelia, there are feeding polyps with tentacles, and reproductive polyps that bud off new medusae. The gastrovascular cavities of all of the individual polyps are all continuous, even though the individual polyps act much like a hydra.
3c. Physalia, the Portuguese man-of-war. Familiar to Florida swimmers, the man-of-war is a colonial hydrozoan with several more levels of complexity than Obelia. One of the siphonophores, the man-of-war has one"polyp" which is modified to be the bell or float. The crest helps to catch the wind, as it otherwise cannot propel itself. Feeding polyps help to digest the food that is caught by the fishing polyps, which have tentacles that can shorten dramatically after catching prey. Also part of the man-of-war are reproductive polyps, although the sexual gonad-bearing individuals remain attached.
4. Class Scyphozoa ("cup animal") (200 species): the true jellyfish. In this class, the medusa is the prominent form. In the life cycle of Aurelia or moon jellyfish, the medusa is sexual and produces egg and sperm. The zygote divides, eventually forming the planula, which settles down and attaches to become a polyp. This polyp develops into a so-called scyphistoma, which buds off young medusae called ephryae before they mature into adults. The scyphozoan adult medusa (which I will call a jellyfish) has more features than the hydrozoan medusa as of Obelia. For example, the jellyfish does not have the velum or shelf-like membrane of the hydrozoan medusa, but does have more canals and features in the bell. The nervous system coordination is such in jellyfish to allow for contractions for jet propulsion. The mesoglea is highly thickened compared to the polyps, accounting for the"jelly" in the jellyfish.
4a. Aurelia, the moon jellyfish. There are four oral arms. These converge at a mouth, with four gastric pouches, each of which contain gonads. Radial canals that expand the gastrovascular cavity run to a circular canal along the margin of the bell. Along this margin are sensory rhopalia. A rhopalium contains sensory structures that detect chemicals, touch, equilibrium, and light. Aurelia feeds on plankton, in contrast to other"fishing" jellyfish.
5. Class Anthozoa ("flower animal") (6,000 species): corals, sea anemones, and sea fans.
The polyp is the predominant form in this class, with the medusa absent. In contrast to hydrozoan polyps, the anthozoan polyps have a pharynx or muscular invagination below the mouth that leads into the rest of the gastrovascular cavity, and several partitions (septa, mesenteries) that divide up the g-v cavity. Coral polyps secrete a calcium carbonate exoskeleton around them, but otherwise tend to be similar to anenomes. Millions of individuals can build up coral reefs (only in warm waters). Other"corals" can be interconnected polyps in the form of sea fans, etc.
6. Class Cubozoa ("cube animal") (15 species): the sea wasps. These cuboidal medusae can be dangerous to humans (the sea wasp Chironex has killed swimmers in Australia). The squarish jellyfish has tentacle(s) at each corner of the bell.
7. Phylum Ctenophora ("comb bearing") (100 species): the comb jellies. Another marine phylum of radially symmetrical animals that look somewhat like jellyfish, but with important differences. They have eight plates of cilia called comb rows (comb plates) that run from between the aboral and oral poles. They have two tentacles with adhesive, not stinging, cells in order to catch prey. Comb jellies are hermaphroditic and their larvae do not undergo a polyp stage.
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Please send comments and questions to: cronewil@hvcc.edu
Copyright 1998 by Wilson Crone
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