GENERAL ZOOLOGY 



stratum by secretions of their posterior adhesive glands. Many species are 

 permanently attached, and a few live in tubes constructed of various materials 

 and immovably fixed to the substratum. 



Although there is no single species of rotifer that may be called "typical," 

 a generalized diagram is helpful in understanding the structural character- 

 istics of the group (Fig. 12.1). The animal is bilaterally symmetrical, and 

 its body may be divided into head, trunk, and tail regions. It is covered 

 by a non-living, proteinaceous cuticle secreted by the epidermis and firm 

 enough to maintain the characteristic shape of the bodv. A variouslv special- 

 ized anterior region bears, in many species, the powerful cilia which in these 

 forms are responsible for food-getting, and in part for locomotion. Water 

 currents set up by the beating of these cilia converge upon the anteroventral 

 mouth, carrying with them microorganisms and particles of debris upon which 

 the rotifer feeds. Other rotifers are provided with various anterior grasping 

 structures which enable them to capture larger prey, such as other rotifers. 

 At the posterior end is the tail, which often bears a pair of toe-like processes, 

 each containing a cement gland. The anus, reduced and non-functional in 

 many species, is posterodorsal. 



The digestive system includes a pharynx containing a grinding organ, the 

 mastax. The components of this organ, the so-called trophi, are especially 

 modified in relation to the food habits of the species, and their morphology 

 has been found useful in taxonomy. The pharynx is also provided with a pair 

 of salivary glands. Posterior to the pharvnx are found, in order, an esophagus, 

 a stomach, a short intestine, and a cloaca. The lining of the digestive tract 

 is often a ciliated epithelium. Digestion appears to be mainly an extra- 

 cellular process occurring under the influence of enzymes produced by 

 glandular cells in the lining of the stomach. Epithelial cells in the stomach 

 and intestine function also in the absorption and storage of the products of 

 digestion. 



Beneath the surface cuticle the body wall is composed of a simple ectoder- 

 mal epidermis; the lining of the digestive tract is of endodermal origin. The 

 tissues between these two layers are mesodermal, including muscle fibers and 

 mesenchyme cells. An ill-defined body cavity is present but is not lined by a 

 peritoneum. There is no circulatory system. A bilaterally arranged pro- 

 tonephridial system is present, opening anterolaterally at a pair of nephridiopores. 

 The nervous system consists of a large ganglion lying dorsally in the head 

 region and smaller ganglia in other parts of the body. Sense organs are 

 generally distributed in the body wall. The system also includes nerve fibers 

 interconnecting the ganglionic masses, sensory fibers from the receptors, and 

 motor fibers innervating the muscle bands. 



Rotifers are dioecious,- that is, the sexes are separate. In one large group 

 males are unknown, however, and reproduction occurs onlv bv partheno- 

 genesis. In other types sexual dimorphism is extreme, the males being much 

 smaller and less well developed than the females. The reproductive system of 

 the female includes compound organs, paired or single, which combine the 



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