Rotatoria (=Rotifera) belong to the smallest metazoa of which over 1000 species have been described, 90% of which inhabit freshwater habitats. They seldom reach 2 mm in body length. Males have reduced sizes and are less developed than females; some measuring only 60 mm. The body of all species consists of a constant number of cells, the different Brachionus species containing approximately 1000 cells which should not be considered as single identities but as a plasma area. The growth of the animal is assured by plasma increase and not by cell division.
The epidermis contains a densely packed layer of keratin-like proteins and is called the lorica. The shape of the lorica and the profile of the spines and ornaments allow the determination of the different species and morphotypes (see 3.4.). The rotifers body is differentiated inTO three distinct parts consisting of the head, trunk and foot (Fig. 3.1.). The head carries the rotatory organ or corona which is easily recognized by its annular ciliation and which is at the origin of the name of the Rotatoria (bearing wheels). The retractable corona assures locomotion and a whirling water movement which facilitates the uptake of small food particles (mainly algae and detritus). The trunk contains the digestive tract, the excretory system and the genital organs. A characteristic organ for the rotifers is the mastax (i.e. a calcified apparatus in the mouth region), that is very effective in grinding ingested particles. The foot is a ring-type retractable structure without segmentation ending in one or four toes.
Figure 3.1. Brachionus plicatilis, female and male (modified from Koste, 1980).