In this article we will discuss about Phylum Gnathostomulida:- 1. Habitat of Phylum Gnathostomulida 2. Features of Phylum Gnathostomulida 3. Phylogenetic Relationship.
Habitat of Phylum Gnathostomulida:
Phylum Gnathostomulida live in the interstitial spaces of marine sands which contain little or no oxygen but sufficient amount of hydrogen sulphide. They have been described from the different parts of the world with maximum population concentration in the east coast of North America.
Examples. Haplognathia (Fig. 14.33A), Gnathostomula (Fig. 14.33B), Problognathia, etc.
Features of Phylum Gnathostomulida:
1. Phylum Gnathostomulida are small, un-segmented, bilaterally symmetrical and acoelomate animals.
2. Generally the body of Phylum Gnathostomulida is cylindrical with the length of the body mostly ranges between 0.5 and 1.0 mm and sometimes extends up to 3 mm.
3. Body of Phylum Gnathostomulida consists of a slightly enlarged head and a tapering posterior tail.
4. Cuticle is absent on the body surface of Phylum Gnathostomulida but single cilium in each epithelial cell rather than several percell.
5. Locomotion with the help of cilia by metachronal beating.
6. Musculature of the body wall cross striated and weakly developed.
7. Phylum Gnathostomulida are triploblastic animals.
8. Phylum Gnathostomulida have no connective tissue.
9. Gut tubular with a muscular pharynx and a pair of cuticular toothed lateral jaws (Fig. 14.33C) but no anus.
10. Circulatory and respiratory systems are absent in Phylum Gnathostomulida.
11. Excretory organs are simple monociliated protonephridia.
12. Most species of Phylum Gnathostomulida are hermaphroditic.
13. Phylum Gnathostomulida have well differentiated reproductive system.
14. Male gonopore opens at the posterior end of the body.
15. Fertilization internal.
16. Spiral cleavage and development direct.
Phylogenetic Relationship of Phylum Gnathostomulida:
The morphological similarities between gnathostomulids and platyhelminthes indicate a closer relationship between the two groups. The features such as ciliated body surface, acoelomate body, spiral cleavage and absence of free-swimming larva indicate a closer relationship with platyhelminthes. Again, the monociliated cells and spirally cleavage in the development resemble with the planula larva of cnidarians.
Remarks:
Ruppert and Barnes (1994) believe that the group Gnathostomulida forms an evolutionary link between a cnidarian-like ancestor and the turbellarians. But Pechenik (2000) has said that the sperm morphology regarding the arrangement of microtubule doublets between gnathostomulids and cnidarians strongly indicate differences.