Phylum Gastrotricha Nervous system Digestive system Parthenogenesis Presentation Best of Luck

1. 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.

4. s

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.

14. References
 Miller_Harley:Zoology,Fifth Edition