THE PHYLUM PLATYHELMINTHES 



207 



from the disintegration of large food particles that at least some pro- 

 teolytic enzymes are secreted. Most of the digestion, however, is intra- 

 cellular. The gastrodermis cells gather up food particles in food vacu- 

 oles. The food vacuoles have not been observed to become first acid 

 and then basic, as they do in other phyla. 



Indigestible remains of food vacuoles are released back into the 

 intestine where, together with fragments that could not be taken up in 

 food vacuoles, they are compressed into solid masses and eventually 

 ejected through the mouth. 



74. Dugesia: Sensation and Movement 



Dugesia is well supplied with sense organs. The "nose," by which 

 the animal explores the physical nature of the bottom on which it is 

 crawling, contains numerous tactile nerve endings. Chemoreceptors 

 (taste-smell) are located in other nerve endings scattered over the body, 

 but are localized especially on the "ears." Each of these is held in a 

 cupped position (Fig. 11.2). Cilia lining the cup beat more vigorously 

 than elsewhere, drawing water from in front of the animal for analysis 

 by the chemoreceptors. 



Planarians capture small prey and are also quick to locate and 

 feed upon dead organisms. When an individual first tastes such food, 

 it raises its head and turns from side to side. The two projections at 

 the sides enable the worm to locate the food, and the worm shortly 

 lowers its head and slides off in the appropriate direction. At frequent 

 intervals it will stop and raise its head again to get new bearings. 



Dugesia is also sensitive to light, generally retreating from it. Each 

 eye (Fig. 11.5) has a pigment cup facing laterally, in the hollow of which 

 are rodlike extensions of visual cells. These rods are arranged radially, 

 and are believed to be stimulated maximally by light tra\eling along 

 their length, since the direction of a light source is very accurately 

 perceived. The bodies of the visual cells, containing the nuclei, lie out- 

 side the cup, and from them a bundle of nerve fibers proceeds to the 

 brain. Such an eye, in which light must first traverse nerve fibers and 

 visual cell bodies before reaching the sensitive rods, is an inverted eye 

 (the human eye is also inverted). Eyes of planarians do not form images. 



^Pigment cup 



Li^hl: sens i-live portion^ 

 / of photoreceptor cell^ 



f- Nuclei oi photoreceptor 

 "^'- ^ cell^ 



■Nerve to brain 



Figure 11.5. Diagrammatic section through the eye of a planarian. Light reaches 

 the sensiti\e elements from the right. 



