PLATYHELMINTHES 



201 



front of, not behind, the retina. This type of eye is easily seen and 

 studied in the common freshwater planarians. In Planaria lugubris, 

 the eye has only two sight cells, while in Planaria lac tea there are 

 thirty. 



Special sensory cells which act as receptors for the appreciation of 

 changes in the composition of the surrounding medium (chemo-sensory 

 receptors) or to changes in the flow of water past the surface of the 

 body (rheotactic receptors) are situated just below the ectoderm. 

 Their endings project through the ectoderm and form the actual 

 receptor organ. The taste receptors are spread uniformly over the 



Ventral 



Fig. 147. Diagrammatic transverse section through the anterior end of a 

 planarian at the level of the cerebral ganglion. After v. Gelei. a and b rheo- 

 tactic sensory cells and their endings; c chemo-serisory cells and e their 

 endings (where the endings of sensory cells of this type occur the cilia are 

 absent) ; d taste cells and their endings; / the two cerebral ganglia in section. 



surface of the body in the Rhabdocoelida, but tend to be more 

 numerous near the mouth. The endings of the taste receptors project 

 among the cilia and are of the same length as these. The rheotactic 

 receptors are confined to certain areas; their endings project among 

 the cilia and are slightly longer than these. Special chemo-sensory 

 receptors with short nerve endings that project only just above the 

 surface of the ectoderm occur in definite areas or grooves on the head. 

 Here the cilia and rhabdites are absent. These areas are known as 

 auricular organs. These sensory organs may also be sunk into pits, 

 which as they are provided with long cilia for driving the water into 

 them, are known as ciliated pits (see Fig. 150). 



