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In this article we will discuss about of Phylum Loricifera:- 1. Features of Phylum Loricifera 2. Phylogeny of Phylum Loricifera 3. Phylogenetic Relationship.
Features of Phylum Loricifera:
1. Phylum Loricifera are microscopic, un-segmented body, and about 200 to 300 µm in length.
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2. Major part of the body called abdomen, covered by 6 overlapping protective plates comprising by rigid cuticle called lorica. So the name of the phylum is ‘Loricifera’ or lorica bearing.
3. Cone-shaped anterior region (introvert) can be retracted into the abdomen and bears recurved spines on the lateral sides, called scalids.
4. Rigid plated cuticle moulted periodically.
5. Mouth is situated at the end of the introvert and encircled by 8 stylets.
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6. Neck composed of several segments.
7. External cilia is absent in Phylum Loricifera.
8. Body cavity of Phylum Loricifera is probably pseudocoelome.
9. Bulb-like pharynx with both circular and longitudinal muscles, and long midgut.
10. Protonephridial excretory system.
11. Gonochoristic (sexes separate).
12. Fertilization is probably internal in Phylum Loricifera.
13. Development not yet known.
14. A free-swimming larval stage called Higgins larva (Fig. 15.23B) is found which differs from the adult having a pair of un-jointed swimming appendages (toes) at the posterior end. The introvert lacks the stylets and thorax lacks the spines.
Three species of loriciferans about 1 mm in length, resemble jellyfish encased in shells, have been discovered in 2010 in the Mediterranean ocean floor at a depth of 3.5 km which was 200 km off the coast of Crete.
The specimens were found in an area which is almost no oxygen and the scientists have reported the first multicellular animals which do not depend on oxygen for breathing and reproduction (e.g., Piciloricus hadalis).
Phylogeny of Phylum Loricifera:
By considering morphological and larval features it may be considered that loriciferans, priapulids, kinorhynchs and nematomorphs may have derived from a common ancestor. Again analysing the many shared features including gene frequency comparisons place the loriciferans under Aschelminthes, and some own peculiar features of adult and larva consider the group as a separate phylum—Loricifera.
Phylogeny of Aschelminthes:
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The phylogeny of aschelminthes is a most controversial issue. Historically the aschelminthes were claimed as monophyletic for some morphological grounds, such as the pseudocoelom, a thick tough cuticle which is often moulted, and adhesive organs. But a single character is not universally present among the different phyla of Aschelminthes.
Pseudocoelom is not present among the members of gastrotrichs, some small-sized nematodes and kinorhynchs. So pseudocoelom is not most distinctive feature for aschelminthes. The body cavity of pseudocoelomates is not lined by coelomic epithelium and has developed as the remnant of the embryonic blastocoel.
Pseudocoelomate condition of different phyla was evolved independently by a number of lineages, so pseudocoelomates are considered as polyphyletic, not monophyletic. Molecular analysis of some pseudocoelomate phyla indicates polyphyletic lineages, not monophyletic.
Pseudocoelomates are considered to be primitive groups because the members of these phyla possess simpler anatomical organization. But the ancestors of aschelminthes were not primitive, they were advanced coelomates. The body cavity of pseudocoelomate may be the secondarily modified coelomic cavity.
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On the basis of morphology, Hyman (1951) has considered that the three groups, such as Rotifera, Gastrotricha and Nematoda have evolved from turbellian-like ancestor and perhaps gastrotricha may be considered to be basal group among three.
Phylogenetic Relationship of Phylum Loricifera:
The relationships among the different phyla consider between the two groups. One group includes gastrotrichs, nematodes and nematomorphs and the other group considers rotifers and acanthocephalans.
Structure of cuticle, pharynx, nervous system and developmental pattern help to establish the relationship between the gastrotrichs and nematodes. Organization of epidermis and development of sperm show the similarities between the Rotifers and Acanthocephalans. The similarities among the rest phyla are not clear.
On the basis of moulting, the different phyla under Aschelminthes are divided into two main groups. One group includes rotifers and acanthocephalans which does not moult. Morphological characters and molecular data indicate that the two phyla are closely related and should be placed in one phylum and may be called Syndermata.
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The other group which includes Nematoda, Nematomorpha, Priapulida, Kinorhyncha, Loricifera, Onychophora, Tardigrada and Arthropoda should be considered as Ecdysozoa that refers to the fact that the members of these phyla shed their cuticle during growth, though several phyla (such as Priapulida, Onychophora, Tardigrada and Arthropoda) are not included under Aschelminthes.
