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In this article we will discuss about Phylum Nematomorpha:- 1. Characters of Phylum Nematomorpha 2. Taxonomic Retrospect of Phylum Nematomorpha 3. Characteristic Features 4. Classification 5. Affinities.
Characters of Phylum Nematomorpha:
Phylum Nematomorpha includes a number of threadlike elongated animals belonging to two families: the Nectonematidae and the Cordiidae. They are free-living in sexual phase and are parasitic in asexual stage in the body cavity of Arthropods.
The family Gordiidae is exemplified by Gordius (Fig. 15.16A). It is a fresh-water or terrestrial form. The outer surface of the body is covered with a cuticle (Fig. 15.16B). The muscles are made up of epithelio-muscular cells. The body encloses four canal-like cavities—one mid-dorsal, one mid-ventral and two lateral in position. These cavities are separated by parenchymatous partitions.
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The alimentary canal is placed in the mid-ventral canal and the lateral cavities contain reproductive apparatus. The nervous system is represented by a greatly thickened pharyngeal nerve ring which is continued posteriorly as a ventral nerve cord. The excretory organs are absent and the sexes are separate. Fertilization is internal. Complete degeneration of the alimentary canal is encountered in the sexual stage.
The family Nectonematidae is represented by the genus Nectonema which is constructed on the same structural plan as Gordius, but differs in minute details. In Nectonema two rows of bristles are present on the side of the body. The reproductive apparatus is simpler than that of Cordius and the ovary is un-segmented. Nectonema is marine.
Taxonomic Retrospect of Phylum Nematomorpha:
1. In the earlier years of taxonomy, the zoologist regarded the horse-hair worms (Gordiaceans) as nematodes related to filarioids.
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2. Von Siebold (1843) created the class Gordiacea on the basis of morphological studies of previous zoologists and also included Mermis, a nematode with Gordiacea.
3. Vijdovsky created also the name Nematomorpha (for Gordiaceans) and separated from nematodes in 1866.
4. Hyman (1951) included Nematomorpha as a class under the phylum Aschelminthes.
Characteristic Features of Phylum Nematomorpha:
1. Body of Phylum Nematomorpha is long, heir-like, un-segmented and worm-like.
2. Length of the body ranging from 0.5 m to 1.0 m.
3. External collagenous cuticle.
4. Cuticle with small papillae, moulted periodically.
5. Absence of locomotory cilia in Phylum Nematomorpha.
6. Presence of only longitudinal body wall muscles.
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7. Gut reduced, non-functional in adult stage.
8. Pseudocoel is mostly fluid-filled body cavity and filled with parenchyma.
9. Circulatory, respiratory and excretory systems are absent in Phylum Nematomorpha.
10. Nervous system of Phylum Nematomorpha consists of cerebral ganglion with an epidermal midventral nerve cord.
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11. Absence of constancy of cell numbers (eutelic condition).
12. Sexes gonochoristic (= dioecious).
13. Simple gonads.
14. The sperm ducts open into a cloaca but there are no penial spicules (Fig. 15.16A).
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15. Fertilization is internal in Phylum Nematomorpha.
16. Bilateral cleavage pattern.
17. The larvae bear a protrusible proboscis which bears spines (Fig. 15.17B).
18. Juveniles parasitize arthropods (e.g., crickets, grasshoppers and other insects) but adults are free-living.
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19. Free-living, aquatic animals or live in damp soil.
Classification of Phylum Nematomorpha:
Phylum Nematomorpha:
The phylum Nematomorpha is divided into two classes:
Class 1. Nectonematoida:
1. Marine and pelagic nematomorphs.
2. Adults measure 20 cm in length.
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3. Cuticle possesses swimming bristles.
4. Fluid-filled capacious pseudocoel.
5. A dorsal and ventral nerve cords.
6. Single gonad only.
7. The juveniles parasitize of marine crustaceans.
It includes a single genus Nectonema.
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Class 2. Gordioida:
1. Generally called hair worms.
2. They are found in ponds, lakes, shallow marshes, streams of slow serpentine movements.
3. Juveniles parasitize insects, like crickets, terrestrial beetles and grasshoppers.
4. Adults measure 1 to 7 cm in length.
5. Swimming bristles absent.
6. Body cavity is a pseudocoel, filled with stellate mesenchymal cells.
7. Only a ventral nerve cord.
8. Presence of paired gonads.
Examples:
Gordius, Paragordius, Chordodes.
Affinities of Phylum Nematomorpha:
A. Affinities of Phylum Nematomorpha with Nematoda:
The nematomorphs bear many characteristics similar to nematodes, kinorhynchs and priapulids.
The following features show resemblances to the nematodes:
1. Un-segmented worm-like body as in nematodes.
2. Collagenous external cuticle in both nematodes and nematomorphs.
3. Only longitudinal muscles in the body wall as in nematodes.
4. Both respiratory and circulatory system absent in nematodes and nematomorphs.
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5. Sexes separate as in nematodes.
6. Nature of body as in nematodes.
B. Affinities of Phylum Nematomorpha with Kinorhyncha:
Similarities:
1. Absence of respiratory and circulatory systems.
2. The cuticle of juveniles in kinorhynchs moults periodically as in nematomorphs.
3. Sexes separate in both groups.
4. Nature of body cavity.
C. Affinities of Phylum Nematomorpha with Priapulida:
Similarities:
1. External cuticle moults periodically in both groups.
2. Extensive body cavity in both groups (The body cavity is either a pseudocoel or eucoelom has not clearly understood).
3. Both groups are gonochoristic.
Remarks:
Considering the above similarities to the three groups the Nematomorpha are more closely related to the Nematoda rather than Kinorhyncha and Priapulida.
