PHYLUM GASTROTRICHA (Or. gastros, stomach i- trichos, hair)
They have about live hundred free- living marine and freshwater species. They live in the space between bottom sediments. Their size is from 0.01 to 4 mm in length. They use cilia on their ventral surface to move over the substrate. The phylum contains a single class divided into two orders. They have following characteristics:
- The dorsal cuticle often contains scales, bristles, or spines.
- A forked tail is often present.
- A syncytial epidermis is present beneath the cuticle.
- Sensory structures are tufts of long cilia and bristles on the rounded head.
- The nervous system includes a brain and a pair of lateral nerve trunks.
- The digestive system is a straight tube. It has a mouth, a muscular pharynx, a stomach-intestine, and an anus. They ingest microorganisms and organic detritus by the pumping action of pharynx from the bottom sediment and water. Digestion is mostly extracellular.
- Adhesive glands are present in the forked tail. They secrete material that attaches the animal to solid objects.
- Paired protonephridia occur in freshwater species but absent in marine species. But their protonephridia are morphologically different from other acoelomates. Each protonephridia possesses a single flagellum. But other acoelomates have cilia in !lathe cells.
- Most of the marine species reproduce sexually. They are hermaphroditic. Most of the freshwater species reproduce asexually by parthenogenesis. The females lay two kits of unfertilized eggs.(a) Thin-shelled eggs: It is hatched into females during favorable environmental conditions(b) Thick-shelled resting eggs can withstand unfavorable conditions for long periods. ‘then they are hatched into females.
There is no larval stage. Development is direct. The young have the same form as the adults.
Fig: Cladogram Showing Evolutionary Relationships among the Classes of Platyhelminthes.