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Gastropods

The Class Gastropoda belongs to the Phylum Mollusca. Gastropods include the sea snails, land snails, slugs, sea hares, limpets, abalones, turbans, tritons and cones. Gastropods are soft bodied animals, with a head, foot, visceral body mass and a mantle, often protected by a shell. But not all gastropods have a shell, such as the beautifully coloured nudibranchs. Many gastropods are able to withdraw into a spiral shell and close off the aperture with a covering door or lid, called an operculum. Others like the limpet, have a cone-shaped shell, and do not have an operculum cover. When first hatched from the egg, the small mollusc creates a protoconch. As it grows, the mollusc adds extra calcareous material onto the outside edge of the shell. Univalve mollusc shells usually grow as an enlarging spiral. The suture divides one whorl from another. Shell sculpture often consists of ribs, bumps or other ornate markings. Ribs may be axial or longitudinal, running parallel to the axis of the body from the anterior to the posterior. Spiral ribs run around the whorls. Other forms of sculpture may be denticles, which are bumps that look like teeth, or pustules that look like pimples. Some shells may have long spines, and other extensions, which may be a camouflage, or assist the mollusc in some other way. During their larval life stages, all gastropods go through a body-change process called torsion, or twisting. The mantle cavity moves from the rear position and comes to rest over the head, facing forward towards incoming water. So inside the mantle cavity are the gills, anus and reproductive glands, which lie directly over the head. This would seem to have some advantages and some disadvantages. The advantage is that the gills face fresh incoming oxygen-charged water. The disadvantage is the excreta from the anus comes out just above the head. But the mantle cavity is designed so the poisonous wastes are quickly dispersed into the water and are quickly washed away from the mollusc. The reproductive gametes and ova are also sent out into the water in this way as well.	Molluscs Chitons False Limpets Limpets Top Shells Nerites Shell-less Molluscs Turbans Tritons Murex Shells Periwinkles & kin Mussels Siphon Shells Bivalves Home Page Taxonomy Biogeography Rocky Shores Tidal Levels Intertidal Zonation Environmental Factors Biological Factors Feeding Relationships Activities Glossary References
Life on Australian Seashores by Keith Davey (C) 2000 Learning Consultant - Media The University of Newcastle email at australian_seashores@hotmail

Chlorophyta: Green Algae

Examples: Chlamydomonas, Spirogyra, Ulva.

Characteristics: Green colour from Chlorophyll a and b in the same proportions as the 'higher' plants; ß-carotene; and various characteristic xanthophylls. Food reserves starch, some fats or oils like higher plants. Thought to be the origin of the higher green plants but there is currently some doubt on this point.

Green algae may be unicellular, multicellular, colonial or coenocytic (composed of one large cell without cross-walls; the cell may be uninucleate or multinucleate). They have membrane-bound chloroplasts and nuclei. Most species are aquatic and are found commonly in freshwater and marine habitats; some are terrestrial, growing on soil, trees, or rocks. Some are symbiotic with fungi giving lichens. Others are symbiotic with animals, e.g. the freshwater coelentrate Hydra has a symbiotic species of Chlorella as does Paramecium bursaria, a protozoan. Chlorella is now grown and sold as a health supplement. A number of freshwater green algae (charophytes, desmids and Spirogyra) are now included in the Charophyta, a phylum of mostly freshwater and terrestrial algae, which are more closely related to the higher plants than the marine green algae.

Asexual reproduction may be by fission (splitting), fragmentation or by zoospores (motile spores). Sexual reproduction is very common and may be isogamous (gametes both motile and same size); anisogamous (both motile and different sizes - female bigger) or oogamous (female non-motile and egg-like; male motile). May have an alternation of haploid and diploid phases. The haploid phases form gametangia (sexual reproductive organs) and the diploid phases form zoospores by reduction division (meiosis). Some do not have an alternation of generations, meiosis occurring in the zygote. There are about 8,000 species of green algae, about 800 of which are marine.

sumber : http://www.seaweed.ie/algae/chlorophyta.lasso

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