LINEUS. 259 



(5) The proboscis nerves which are given off one 

 on each side of the ventral ganglia and pass thence into 

 the proboscis. Inside this structure they soon spread out 

 and fuse to form a nervous sheath investing the probos- 

 cidial epithelium in the retracted state of the organ. 



On the nerves of the peripheral system are found some 

 nuclei, but these probably belong to neuroglial, not 

 ganglion cells. 



SENSE ORGANS. 



The ciliated cells of the epidermis doubtless function as 

 sensory cells, though whether the sensory elements can be 

 distinguished apart from the ordinary ciliated cells has 

 not been determined in the case of Lineus gesserensis. 

 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 

 by mobile lips, reaching from the tip of the head nearly 

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

 wards. At the posterior extremity of each of these head 

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

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

 ward and then a forward course (PI. III., fig. 3, ce.) t lies 

 for its whole extent in close proximity to the hinder 

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

 glands. The first set (PI. I., fig. 3, and PI. III., fig. 3, 

 acg.) 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 hig-h thin columnar cells devoid of 

 glands, but behind the opening of the posterior gland the 

 epithelium of the ciliated canal becomes greatly changed. 



