Phylum Gnathostomulidia (Meaning jawed mouth)

1. This presentation will probably involve audience discussion, which will create action items. Use PowerPoint to keep track of these action items during your presentation • In Slide Show, click on the right mouse button • Select “Meeting Minder” • Select the “Action Items” tab • Type in action items as they come up • Click OK to dismiss this box • This will automatically create an Action Item slide at the end of your presentation with your points entered. PhylumGnathostomulidia(Meaning jawed mouth) Christine Chan

2. Introduction • Discovered about 45 years ago • At present the group contains 94 described species,distributed among 25 genera • Most are less than 1mm long • In worm concentrations 1kg of sand can contain 6,000 individuals • Worm aggregations occur in deeper sediment where there are higher concentrations of hydrogen sulfide • They inhabit marine interstitial environments, the greatest species diversity in tropical coralline sand

3. Body Wall/Support • The gnathostomulid body can be divided into a head and a trunk, and in some cases a small offset tail. • They are acoelomates which are triploblastic animals lacking an internal body cavity • They have bilateral symmetry

4. Body Wall/Support • The integument (The outer protective layer of tissue, which covers an animal) is totally covered with locomotory cilia that all arise from the epithelian cells which are monociliated • Loose mesenchyme below epidermis, with some thin muscle bands • Movement is achieved through ciliary gliding, swimming and contortion of the body

5. Internal Workings • Mouth on ventral surface -jaws are located in the pharynx • The gut is saclike or a simple tube • Their gas exchange is done by diffusion • Nervous System • Brain, pharyngial ganglion, and three pairs of epidermal nerves • Lots of sensory pits and cilla located on head • They lack any sort of specialized circulatory or respiratory systems • Their excretory organs are simple protonephridia

6. Feeding • They feed on encrusted bacteria and fungi found on sand grains and other sediments • They use their basal plate to scrape up the food • A very short esophagus extends from the pharynx into the gut. • Then their jaws chop up the food before passing into the gut

7. Jaws

8. Feeding • An anus is apparently missing but it has been shown that some species might have indications of a temporarily present anal pore. • A coelom is absent and fluid wastes are excreted via monociliated protonephridia

9. Reproduction • Most gnathostomulid species are hermaphroditic • The egg is fertilized internally and then released • The single egg later escapes from the body by breaking directly through the body wall • Cleavage is spiral • The zygote however, develop directly into small worms • However there are some species that have separate sexes, and males impregnate females with by piercing her hypodermically with their stylet

10. Rastrognathia macrostoma http://hooge.developmentalbiology.com/gnatho/

11. Problems in phylogeny • They resemble the turbellarian flatworms so are considered closely related to they platyhelminths. • Monociliated epidermis, with a single cilium per cell indicates that they are closely related to planula larvae of cnidarians. • Some think that the Gnathostomulids form the missing link between the Cnidarians and the Turbellarians. • There is also people that claim that they are related to the annelids.