1 98 LANGDON. [Vol. XI. 



brane of the epidermis. This ovoid may be broad or narrow, 

 and is usually more or less irregular; the base is sometimes 

 distorted by the presence under it of a bundle of nerve fibres. 

 The greatest width is usually just above the base, occasionally 

 it is at the base. In a cross section of a sense-organ its out- 

 line rarely appears as a circle, but it is usually a little flattened 

 or otherwise distorted. 



The sense-organs in the middle zone of an anterior meta- 

 mere vary in height from 80 to ioo/a. At its summit such an 

 organ may be from 18 to 28/x wide; at the widest part, from 

 40 to 60 [I wide. 



The lateral limit of each organ is clearly defined by the layer 

 of supporting cells around it. This is shown in longitudinal 

 sections, in which the sense-organ is clearly outlined on each 

 side by one or two supporting cells, which follow the outline 

 of the organ and which are much flattened, evidently by 

 pressure from within the sense-organ. In a cross section of 

 a sense-organ, the inner (in relation to the organ) walls of 

 these adjacent supporting cells are found to form a continuous 

 membrane around the organ. It is thus seen that the cells of 

 the sense-organs are enclosed in a cavity whose walls are formed 

 by one or two layers of supporting cells which differ from the 

 other supporting cells only in their flattened form. These may 

 be called the covering cells of the sense-organ. 



Within the receptacle thus formed by the covering cells, lie 

 the two kinds of cells belonging to the sense-organ, small basal 

 cells and long slender cells which extend from the basal mem- 

 brane of the epidermis through the cuticula. The small cells 

 are sometimes evenly distributed along the base of the sense- 

 organ, and sometimes collected into a little group in the center 

 of the base or toward one side. Each cell is irregularly rounded 

 and almost filled by a rounded nucleus — in fact, it does not 

 differ from the small cells of the basal layer of the epidermis. 

 I have not been able to determine the function of these basal 

 cells of the sense-organ; it seems to me possible that they are 

 the cells which are to produce new sense-cells, but I have 

 found no intermediate forms. The long cells, which are the 

 true sense-cells, occupy the main part of a sense-organ; there 



