ORGANS OF SPECIAL SENSE 169 



Further, this middle or lens-shaped region, which in both the 

 living and fixed specimen appears to be a highly refractive body, 

 is the only essential feature that makes this simply formed eye 

 different from that described by Hesse ('97) for Planaria torva. 



Now to return to the cytology of the retinular cell, the mesial 

 region or the rhabdome is a low sugar loaf-shaped body that 

 fills the pigmented cup of the accessory cell. In the fixed con- 

 dition this region presents a finely granulated appearance (fig. 

 8, Rh). 



As stated l)efore, we consider the ciliated pits the chief organs 

 of special sense. These organs open on the ventral side and are 

 disposed laterally and ventrally to the thick nerve commissure 

 which joins the two dorsal ganglia (text fig. C, CP). When 

 the animal is crawling upon a surface it seems to make numer- 

 ous exploratory movements by raising and lowering the anterior 

 sixth of its body. Thus we have a crawling animal with ven- 

 trally disposed ciliated pits which makes its exploratory move- 

 ments by raising and lowering the anterior end of its body. It 

 is well to compare the conduct of this animal with that of Micro- 

 stoma caudatum Leidy with reference to the position of their 

 respective ciliated pits. Microstoma caudatum is a free-swim- 

 ming animal and has laterally disposed pits. As we have shown 

 ('12) this animal makes exploratory movements by moving its 

 anterior end from side to side. Likewise we gave experimental 

 evidence to show that these exploratory movements were made 

 in order to test the surrounding medium. Thus we see by the 

 comparison of the two rhabdocoeles that the method in which 

 they test the surrounding water conforms to the position of their 

 ciliated pits. 



The remaining fourteen cells of the eighteen concerned in the 

 organs of special sense are involved in the structure of the two 

 ciliated pits. These organs are invaginated regions of the ven- 

 tral epidermis, directed obliquely, posteriorly and mesially. Of 

 the seven cells that form each pit, six form a syncytium with 

 two rows of three nuclei each. One row is disposed dorso- 

 ventrally and forms the lateral wall of the pit, while the other 

 row forms a similar mesial wall (figs. 6 and 7). The general 



