Species Differentiation and Ecological Relations of Planarians

Abstract

The planarians, or freshwater triclads, belong, in the zoological classification, to the class Turbellaria (free-living flatworms) of the phylum Platyhelminthes. Thus they are among the lowest, or most primitive, Metazoa with bilateral symmetry and a centralized nervous system. This primitiveness in their structure and, presumably, physiology makes them eminently suitable for the study of many fundamental biological phenomena of a general nature, applicable in many ways to higher animals as well. They have long been favored experimental animals in laboratories. The classical studies of the principles of regeneration (T. H. Morgan, C. M. Child, and many other investigators, see Brøndsted, 1955) were made on planarians. Other investigations in which planarians played a prominent role were studies of the differential action of radiations on various types of cells and tissues (C. R. Bardeen, W. C. Curtis, F. Stéphan-Dubois, etc.), effects of pharmacological and carcinogenic agents (many investigations widely scattered in the literature), the morphogenetic interrelations of various organs during their development (E. Wolff and his collaborators), and relations of cytogenetics to speciation (M. Benazzi and his school). During the last decade we have seen a very intensive upswing in behavioral studies performed on planarians, centering on the mechanisms and possible biochemical correlates of learning and memory, which are the topics of the present symposium.