46 JOHNS HOPKINS UNIVERSITY MORPHOLOGICAL MONOGRAPHS. 



part of the nervous system. As to function, the idea that they serve to 

 give perception of space relations suggests itself as readily as any other 

 hypothesis. 



We come now to the consideration of the terminal knob of the clubs, 

 the sensory portion proper. A complete and detailed account of the 

 complex structure of these organs would fill many pages and involve 

 much useless repetition. Claus ('78) has described them with accuracy, 

 but not in great detail, and since then Schewiakoff ('89) has given a 

 careful general description and has supplemented Claus's work by 

 observations upon the finer structure made with the aid of more recent 

 technique. It seems in place for me, therefore, to give in the briefest 

 possible way a general idea of their structure, and to pass then at once to 

 the points in which my work has led me to different conclusions from 

 those of Claus and Schewiakoff. In brief, then, the knob of the sensory 

 club consists of a thick, complex mass of nerve fibres, more or less 

 imbedded in which lie the special sensory organs, surrounding the 

 ampulla-like terminal enlargement of the canal. The surface between 

 the special organs is covered with less specialized sensory epithelium. 

 The sensory organs are seven in number. Of these, four are simple 

 invaginations of the surface epithelium arranged in two pairs symmet- 

 rically to the median line in the proximal end of the knob (the end where 

 the stalk enters) and having pigment developed in the cells so invagi- 

 nated, while the space of the invagination is filled with a gelatinous refract- 

 ing secretion. These are considered simple eyes. Two more of the organs 

 are complex eyes situated on the median line of the inner surface of the 

 knob, the upper one smaller than the lower, but having almost exactly 

 the same structure. Each has a cellular lens over which extends a super- 

 ficial, corneal layer of cells ; below the lens a refractive "vitreous body"; 

 and below this a retina with pigmented cells. The seventh organ is the 

 crystalline sac, which lies almost at the end of the knob opposite to the 

 stalk and contains a large concretion. In view of the fact that the 

 sensory clubs in toto have been abundantly figured by Claus and 

 Schewiakoff, it is my intention to give but one simple figure of the 

 general relations, and I justify that one in that it was made from the fresh 

 material. Fig. 57 is a camera sketch of the outlines given by a sensory 

 club seen in optical section from the side. The smaller upper and the 

 larger lower complex eyes which are situated on the mid-line, are seen in 

 profile, while the two small simple eyes give the outlines that they would 

 in a surface view of their side of the knob. Of course it is understood 



