GENERAL ZOOLOGY 



Fig. 1 1 .2. Food detection and feeding 

 by pianarians. A, path of a planar- 

 ian reacting to the presence of food, 

 shown at the point x; the numbers 1 -7 

 mark successive positions of the worm 

 after it enters the zone of difTusion 

 (circle) of juices from the food, fi, 

 diagrammatic representation of the cil- 

 iary currents maintained over the sen- 

 sitive auricles of Dendrocoelum lacleum. 

 C, pianarians feeding; note the 

 extended proboscides (pharynges) 

 applied to the bit of meat upon which 

 the animals are feeding. (A, from 

 R. Pearl, 1903, Quarterly Journal of 

 Microscopical Science, vol. 46; B, after 

 P. Steinmann, from E. Bresslau in W. 

 Kiikenthal and T. Krumbach, 1933, 

 Handbuch der ^oologie; C, from a draw- 

 ing by George T. Kline.) 



Acoela ("without cavity"), Alloeocoela ("different cavity"), Polycladida 

 ("many branches"), Tricladida ("three branches"), and Rhabdocoela ("rod 

 cavity"). The acoels and polyclads are exclusively marine; the alloeocoels 

 are predominantly marine but are found also in brackish and fresh water; 

 the triclads and rhabdocoels occur chiefly in fresh water. The turhellarian 

 body is usually flattened as in most Platyhelminthes and is differentiated 

 anteriorly into a region which may be called a head. Cilia are found 

 upon the entire external surface of some turbellarians but usually occur only 

 in limited areas. The name Turbellaria (lurbella, a small whirlpool) refers to 

 the effect produced by the action of these cilia (Figs. 11.1, 11.2). The 

 mouth may be located in any position along the ventral midline, from the 

 anterior to the posterior end, in different species; the usual and apparently 

 primitive position is near the center of the body. As in coelenterates, the 

 body is covered externally by epidermis, and the digestive cavity is lined by 

 gastrodermis. Between these epithelial layers is a spongy meshwork of 

 mesenchyme, in which are distributed muscle fibers, gland cells, and wander- 



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