Being Transactions of the S. Ajr. Phil. Society. Vol. XVll. 155 



may also be associated with its sepaiation from the epidermis at 

 this point. 



The most striking featm-e of the nervous system, however, is to be 

 found in the character and position of the nerve ring. It is sunk 

 in a comparatively deep involution of the epidermis, which is so 

 intimately connected with the nervous system that it is most con- 

 veniently dealt with here. This involution or fold of the epidermis 

 passes from the oral side of the animal round on each side, follow- 

 ing the course of the nerve ring, till, on reaching the lophophoral 

 opening, in or near which are situated the anal and nephridial 

 apertures, it is continued round the body, passing outside these 

 organs. 



The fold can readily be seen with a low magnifying glass. It 

 varies greatly in depth ; outside the anus it is a mere ridge ; it 

 is deepest at the sides of the body, being about one-fourth of its 

 diameter. Anteriorly, i.e., in the oral region, it becomes somewhat 

 shallower. 



The general character of the histological elements of this fold is 

 indicated in fig. 4. The external part is identical in structure with 

 the general epithelium of the body — a layer of deep epithelium cells, 

 including glandular cells. Towards the tip of the fold, however, 

 these cells become smaller and lower, and at the tip somewhat 

 abruptly change in character and become low, square, with com- 

 paratively large, round nuclei, and without distinct cell demarcations. 

 These are continued down to the bottom of the fold, where they 

 once more abruptly change into deep elongate cells, with punctated 

 nerve substance. The nervous epithelium of the nerve ring does 

 not, therefore, participate in the folding. 



In some sections of the fold a few cells occur betw^een the 

 cubical epithelium and the epidermis, and apparently a space which 

 is not coelomic. The existence of a space is doubtful, however, and 

 may be an artifact. 



In life, the small cubical epithelium of the fold is applied to the 

 nervous epithelium, the lip of the folding not standing out as shown 

 in the figure, which is from a specimen (B) killed and fixed in 

 sublimate. In the other specimen (A), which was treated in practi- 

 cally the same w^ay (fixed in sublimate with 1 per cent, acetic acid) 

 before removal from the tube, the lip of the fold was closely applied 

 to the body. 



The epithelium of the nerve ring is not entirely covered by the 

 fold, even when closely applied to it. In the region of the organs, 

 which for want of a better name I have called " olfactory organs," it 

 extends beyond the fold and is freely exposed. The nerve ring is 



