In this article we will discuss about Phylum Gastrotricha:- 1. Taxonomic Retrospect of Phylum Gastrotricha 2. Characteristic Features of Phylum Gastrotricha 3. Habitat 4. Affinities.
Taxonomic Retrospect of Phylum Gastrotricha:
1. The gastrotrichs were previously united with rotifers by Ehrenberg in 1838.
2. Metschnikoff (1864) separated the gastrotrichs from rotifers and gave the name Gastrotricha.
3. Butschli (1876) combined the gastrotrichs and echinoderids and proposed a common name Nematorhyncha.
4. Zelinka (1889) united rotifers and gastrotrichs under the phylum Trochelminthes.
Characteristic Features of Phylum Gastrotricha:
1. Phylum Gastrotricha is microscopic, un-segmented, worm-like animals.
2. Body length ranges 50-1000 µm.
3. Cephalic region slightly swollen and caudal region forked with glands.
4. Dorsal and lateral surfaces of Phylum Gastrotricha is covered with spines, scales and warts.
5. Ventral surface with abundant cilia, hence called Gastrotricha.
6. Head with a number of sensory bristles.
7. Locomotion by cilia and muscular action. They glide over the bottom on ventral cilia.
8. Phylum Gastrotricha has multi-layered cuticle.
9. Epidermis with monociliated cells with gland cells and glandular adhesive tubes in some species as are gnathostomulids.
10. Circular and longitudinal muscles are present in the body wall of Phylum Gastrotricha.
11. The space between the body wall and gut is filled with loose organs and mesenchyme that indicate the acoelomate condition. Blastocoel forms during development but does not persist in the adult stage.
12. Digestive tract complete; mouth surrounded by bristles; pharynx triradiate and muscular.
13. Circulatory and respiratory systems are absent in Phylum Gastrotricha.
14. Nervous system consists of two cerebral ganglionic masses at the anterior of the pharynx with two lateral nerve cords.
15. Excretory organs of Phylum Gastrotricha are protonephridia with ciliated flame cells.
16. Phylum Gastrotricha are eutelic animals.
17. Hermaphrodite or parthenogenetic with simple gonads. Marine species are mostly hermaphroditic and freshwater species are parthenogenetic.
18. Fertilization is internal in Phylum Gastrotricha.
19. Development with determinate cleavage as in rotifers and other aschelminthes.
20. Blastopore forms both mouth and anus.
21. No larval stage occurs in the life cycle of Phylum Gastrotricha.
Habitat of Phylum Gastrotricha:
They are found in marine and freshwater habitats and the marine species inhabit the interstitial spaces of benthic sediments.
The phylum Gastrotricha is divided into two orders:
(i) Macrodasyida and
(ii) Chaetonotida (Fig. 15.1).
The traditional rank of the order has been maintained.
Order 1. Macrodasyida:
(i) Marine and estuarine animals. They inhabit the interstitial water of sandy and muddy sediments.
(ii) Body with adhesive tubes at the anterior and posterior side of the body.
(iii) Pharyngeal pores present.
(iv) Several pairs protonephridia present.
Macrodasys, Turbanella, Platydasys, Pseudostomella.
Order 2. Chaetonotida:
(i) Mostly freshwater species.
(ii) Body with adhesive tubes at the posterior side of the body.
(iii) Pharyngeal pores absent.
(iv) One pair protonephridia with ciliated flame cells present.
(v) Most freshwater species are parthenogenetic females, lacking male sexual organs.
Chaetonotus, Halichaetonotus, Lepidodermella.
Affinities of Phylum Gastrotricha:
Similarities with Rotifers:
1. Simple musculature.
2. External ciliation.
3. Forked caudal region.
4. Presence of adhesive gland,
5. Protonephridia with flame cells.
6. Eutelic animals.
The similarities do not bear any close relationship between the two groups.
Similarities with Nematoda:
1. Cuticular structure.
2. Structure of the pharynx.
3. Muscle innervation.
4. Similar nature of the brain.
5. Cleavage pattern.
From the above similarities, the gastrotrichs are more closely related to Nematoda than other groups of invertebrates. Hyman (1951) stated that gastrotrichs, nematodes along with rotifers would have arisen from a turbellarian-ancestor. Ruppert and Barnes (1994) considered that gastrotrichs and Nematoda both constitute a monophyletic group.
Some Indian gastrotrichs:
1. Chaetonotus similis,
2. Chaetonotus brevispinosus and
3. Chaetonotus trachyneusticus.