Invertebrate Zoology Rotifera


The rotifers derived their name from the characteristic ciliated organ, the corona, around lobes on the heads of these animals.  The cilia of the corona do not beat in synchrony; instead, each cilium is at a slightly earlier stage in the beat cycle than the next cilium in the sequence.  A wave of beating cilia thus appears to pass around the periphery of the ciliated lobes and gives the impression of a pair of spinning wheels.  (The rotifers were first called “wheel animalicules.”)


Rotifers are small animals (0.1 to 3 mm in length) that are abundant in most freshwater habitats; a few (less than 10%) are marine.  The approximately 2,000 species are divided into three classes.  The body has about one hundred cells, and the organs are eutelic.  Rotifers are usually solitary, free-swimming animals, although some colonial forms are known.  Others occur between grains of sand. The only hard parts of rotifers are their jaws. Because of their mostly soft bodies and small size, rotifers are not commonly favored for fossilization.


Based on certain morphological similarities, rotifers and acanthocephalans (the parasitic worms in the phylum Acanthocephala) have long been considered close relatives. Recent comparisons of 18S rRNA gene sequences provide further evidence of close relationship between these two groups.


Phylum Rotifera

1.      Triploblastic, bilateral, unsegmented, pseudocoelomate

2.      Complete digestive system, regionally specialzed

3.      Anterior end often has a ciliated organ called a corona

4.      Posterior end with toes and adhesive glands

5.      Well-developed cuticle

6.      Protonephridia with flame cells

7.      Males generally reduced in number or absent; parthenogenesis is common


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