LECTURE NOTES ON SALIENT EATURES OF PHYLUM ROTIFERA

Rotifera are, popularly known as “wheel animalcules" due to  their peculiar movement similar to the rotation of a wheel. They are actively swimming,microscopic aquatic animals, most common in freshwater habitats, such as ponds, lakes, streams, marshes .films of water, etc. Some are marine. Majority of them are benthic forms, but a few are planktonic species. They are relatively short living, and their life span ranges between a few days and a few weeks. They are well adapted to to withstand desiccation and to survive drought and hostile climatic conditions. Therefor they live in moist or partially wet terrestrial habitats, such as the cracks and crevices of rocks, fissures on land, etc. Rotifers are bilaterally symmetrical. unsegmented and pseuclocoelomate metazoans, with ciliated trochal  disc, syncytial epidermis, complete alimentary canal, specialized jaws, muscular pharynx and flame bulb type nephridia, and without circulatory  and respiratory organs. The salient features of the phylum are the following: .

(i) They have microscopic body  partially differentiated into head, trunk and tail or foot.

(ii) Anterior head region is broad with a  central disc, called apical field,which  is encircled by a ciliary zone, known as corona (rotatory organ or wheel organ. Corona may be partially subdivided into retractile, locomotor and food collecting discs called trochal discs.

(iii) Body is covered by a cuticle, formed of a glycoprotein, without-chitin or collagen. The cuticular covering sometimes becomes a hard and thick encasement, called lorica.  Organism with lorica are called loricates. and those without lorica are called illoricates.

(iv) Epidermis is mostly syncytial, except in the region of corona.

(v) Peri-visceral cavity is a spacious pseudocoel.

(vi) Pharynx is modified into a muscular mill, called mastax which  is provided with a masticatory apparatus or dental mill .Phrynx is   composed of hard and jaw-like cuticular structures called trophi or internal jaws. Mastax and trophi are characteristic of rotifers.

(vii) They  are ammonotelic. Excretory organs are protonephridia with flame bulbs.

(viii) Nervous system consists of a bilobed brain and several radiating nerves.

(ix) Sexes are separate. Sexual dimorphism is distinct .Male being smaller and almost degenerate. Eggs are of two kinds, namely syngamic eggs and parthenogenetic eggs. The former develops through fertilization, and the latter through parthenogenesis  without fertilization. In some rotifers  males are less numerous than females. In still others, males are altogether absent, and reproduction is exclusively parthenogenetic.

 (x) Many rotifers have the powers for cyclomorphosis and cryptobiosis. Cyclomorphosis is the seasonal change in the form, shape and proportion of the body. Cryptobiosis or “hidden life” is a death-like state of suspended animation which protects the animal from adverse environmental conditions.