In this article we will discuss about Aschelminthes:- 1. Characters of Aschelminthes 2. Taxonomic Retrospect of Aschelminthes 3. Characteristics.
Characters of Aschelminthes:
Aschelminthes are a heterogenous assemblage of a number of groups of animal, such as Gastrotricha, Nematoda, Nematomorpha, Rotifera, Acanthocephala, Kinorhyncha and Loricifera and most of which contain the pseudocoelom as the body cavity.
The pseudocoelom is the fluid-filled body cavity and not lined with mesodermal epithelium and also not developed by schizocoely and enterocoely. It represents as a remnant of blastocoel. The groups of animals containing the pseudocoelom are called pseudocoelomata.
Taxonomic Retrospect of Aschelminthes:
(i) Gegenbaur created the group Nemathelminthes in 1851 which included nematodes, acanthocephalans and gordiaceans.
(ii) Grobben introduced the term Aschelminthes in 1910 instead of Nemathelminthes. Aschelminthes included the different groups of pseudocoelomate animals at the time of introduction of the term Aschelminthes.
(iii) Hyman (1951) supported Aschelminthes as phylum and several classes placed under it. She included Priapulida within Aschelminthes as a class between the Kinorhyncha and the Nematoda.
(iv) Shopeero (1961) removed the Priapulida from Aschelminthes because the priapulids contain the primitive grade coelom.
(v) Marshall and Williams (1972) considered also Aschelminthes as a distinct phylum and included 5 classes under it.
These are:
Phylum Aschelminthes:
Class Nematoda
Class Nematomorpha
Class Rotifera
Class Gastrotricha
Class Kinorhyncha
Recently various classes have been included under the Aschelminthes and these classes have been upgraded into phyla and Aschelminthes is treated as a group without mentioning any taxonomic rank.
(vi) Barnes (1980) elevated the above mentioned classes into phyla rank and also included Acanthocephala and Gnathostomulida.
Recent zoologists have removed Gnathostomulida as a separate phylum and has placed in the acoelomate groups. The lack of body cavity, cuticle and presence of spiral cleavage in development of gnathostomulids also suggest a closer resemblance with Platyhelminthes.
(vii) Ruppert and Barnes (1994) have added Tardigrada which was an allied group of Arthropoda. The tardigrades have included in the Aschelminthes for the pseudocoelom, structure of pharynx (myoepithelium) and constant number of cells in certain organs (eutelic condition).
(viii) Anderson (1998) has separated Tardigrada from pseudocoelomate groups and included with Onychophora for shared characters-like cuticle, muscle structure, appendages, etc.
Brusca and Brusca (2003) used the term “Blastocoelomate groups” instead of pseudocoelomate groups.
Characteristics of Aschelminthes:
1. Aschelminthes are mostly aquatic (marine, e.g., kinorhynchs and loriciferans or freshwater, e.g., Rotifera and some other members) but some are terrestrial. Aschelminthes include both free-living as well as parasitic animals.
2. Body of Aschelminthes is bilaterally symmetrical, triploblastic and un-segmented except Kinorhyncha.
3. Body size mostly small, some microscopic; while others ranging from few milimetres to a metre in length.
4. No well-formed head.
5. Cuticle is a thick, non-cellular and flexible body covering and moulted periodically except Rotifera and Acanthocephala.
6. Body wall musculature of Aschelminthes is only longitudinal muscles, but longitudinal and circular muscles are present in some phyla (e.g., Gastrotricha and Acanthocephala).
7. Adhesive glands are present in most phyla of Aschelminthes.
8. Cilia and muscles are used for locomotion.
9. Body cavity of Aschelminthes is either pseudocoel or some groups are acoelome (e.g. gastrotrichs, most smaller nematodes and kinorhynchs).
10. A phenomenon called eutely (Constancy of numbers of cells in the adult stage of a specific species. The size of the body is increased through the growth in the size of cells, not through the number of cells) is present in most phyla.
11. Non-muscular straight gut with a mouth and an anus. Pharynx is muscular and highly specialized in some cases.
12. Respiratory and blood vascular systems are absent in Aschelminthes.
13. Nervous system of Aschelminthes consists of circumenteric nerve ring with longitudinal nerves.
14. Protonephridia are the typical excretory organs.
15. Mostly dioecious (= gonochoristic).
16. Gonads and gonoducts single or double.
17. Cleavage strongly determinate in most phyla.