18 



(5) The proboscis nerves which are given off one 

 on cacli side of the vontial g'ang'lia and pass thence inio 

 Uio piohoscis. Insidi> this stnictuie they soon spread out 

 and fuse to form a ncivous sheath investing- the prohos- 

 cidial epithelium in the retracted state of the organ. 



On the nerves of the ])eripheral system are found some 

 nuclei, hiit these prolnihly helong to neuroglial, not 

 ganglion cells. 



S]:XSE ORGANS. 



The ciliated cells of the epidermis douhtless function as 

 sensory cells, though whether the sensory elements can be 

 distinguished a])art from the ordinary ciliated cells has 

 not been determined in the case of Linens f/esserens/'s. 

 Some observers, however, have been able to distinguish 

 such cells in other species. Apart from these, three forms 

 of sensory organs are found in the present species. 



(1) The cerebral organ. It has already been noticed 

 that on either side of the head there is a groove bounded 

 l)y mobile lips, reaching from the tip of the head nearly 

 to the mouth region, and deepening as it passes back- 

 Avaids. At the posterior extremity of each of these head 

 slits (PI. IV., fig. 1, hs.) is a small aperture marking the 

 opening of a fine blind canal which, taking first a back- 

 ward and then a forward coui-se (PI. III., fig. •!, re), lies 

 for its whole ext(Mit in clo.-e pioximity to the hinder 

 portion of the dorsal ganglion. Into it open two sets of 

 glands. The lirst set (PI. I., fig. -•;, and PI. III., fig. ••), 

 "(■</.] opens into the canal immediately after its com- 

 mencement, the second set a little further back (PI. III., 

 fig. 3, peg.). Up to this point the epithelial lining of 

 the canal consists of high thin columnar cells devoid of 

 glands, but behind the opening of the posterior gland the 

 e])itlielium of the ciliated canal becomes greatly changed. 



