Phylum Gastrotricha
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Gastrotricha is a phylum containing about 500 free-living marine and freshwater species that inhabit sediments, ranging from 0.01 to 4 mm in length. These organisms have a complex anatomy including a nervous system with a brain, a digestive system for feeding on microorganisms, and unique reproductive methods, primarily through sexual reproduction in marine species and asexual parthenogenesis in freshwater species. Recent studies suggest a distant phylogenetic relationship with acoelomates and ancient flatworms, indicating evolutionary links and divergence in lifestyle.
Phylum Gastrotricha
- 2. Phylum Gastrotricha The gastrotrichs (gas-tro-triks) (Gr. gastro, stomach trichos, hair). They are members of a small phylum of about five hundred free living marine and freshwater species that inhabit the space between bottom sediments. They range from 0.01 to 4 mm in length. Gastrotricha use cilia on their ventral surface to move over the substrate.
- 3. Cont…. The phylum contains a single class divided into two orders The dorsal cuticle often contains scales, bristles, or spines, and a forked tail is often present. A syncytial epidermis is beneath the cuticle. Sensory structures include tufts of long cilia and bristles on the rounded head.
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- 5. Cont…. Nervous System: Nervous system includes a brain and a pair of lateral nerve trunks. Digestive System Digestive system is a straight tube with a mouth, a muscular pharynx, a stomach-intestine, and an anus. The action of the pumping pharynx allows the ingestion of microorganisms and organic detritus from the bottom sediment and water.
- 6. Cont… Digestion is mostly extracellular. Adhesive glands in the forked tail secrete materials that anchor the animal to solid objects. Paired protonephridia occur in freshwater species, rarely in marine ones. Gastrotrich protonephridia, however, are morphologically different from those found in other acoelomates. Each protonephridium possesses a single flagellum instead of the cilia found in flame cells.
- 7. Cont.. Most of the marine species reproduce sexually and are hermaphroditic. Most of the freshwater species reproduce asexually by parthenogenesis. Parthenogenesis: the females can lay two kinds of unfertilized eggs. Thin-shelled eggs hatch into females during favorable environmental conditions. whereas thick-shelled resting eggs can withstand unfavorable conditions for long periods before hatching into females.
- 8. Cont… There is no larval stage; development is direct, and the juveniles have the same form as the adults.
- 9. Furthur Phylogenetic Consideration Zoologists who believe that the platyhelminth body form is central to animal evolution envision an ancestral flatworm similar to a turbellarian. For example: A cladogram emphasizing the uniqueness of the tegument as a synapomorphy. They shared, evolutionarily derived character used to describe common descent among two or more species) uniting the Monogenea, Trematoda, and Cestoidea.
- 11. Cont.. Recent molecular data suggest that the acoelomate flatworms should not be considered members of the Phylum. Platyhelminthes and that they are very close to the first ancestral bilateral animals. More conclusive evidence links the parasitic flatworms to ancient, free-living ancestors.
- 12. Cont… The free-living and parasitic ways of life probably diverged in the Cambrain period, 600 million years ago. The first flatworm parasites were probably associated with primitive molluscs, arthropods, and echinoderms. They must have acquired the vertebrate hosts and complex life cycles.
- 13. Cont… The gastrotrichs show some distant relationships to the acoelomates. For example: many gastrotrichs lack a body cavity and are monoecious and small, and their ventral cilia may have been derived from the same ancestral sources as those of the turbellarian flatworms.