THE EVIDENCE OF THE AUDITORY APPARATUS 365 



readily than a blood- coagulum, yet in the sense of being structure- 

 less it resembles a coagulum. 



The enormous number of these organs crowded together over 

 the whole flat surface of the nabellum produces a very striking 

 appearance when viewed on the surface. Such a view presents an 

 appearance resembling that of the surface-view of the branchial sense- 

 organs ; in both cases the surface is covered with a great number 

 of closely set circular plaques, in the centre of each of which is seen 

 a well-marked pore. The circular plaques in the case of the nabellum 

 are much smaller than those of the branchial sense-omans, and 

 clearly are not protrusible as in the latter organs, the appearance as 

 of a plaque being due to the ring of thickly-set canaliculi round the 

 central tubule, as already described. When stained with methylene 

 blue, the surface view of the nabellum under a low power presents 

 an appearance of innumerable circular blue masses, from each of 

 which springs a fine bent hair, terminating in a pore at the surface. 

 "The blue masses are the homogeneous substance (cap.) of the bulgings 

 seen through the transparent external layer of chitin, and the hairs 

 are the terminal part of the chitinous tubules. Patten has repre- 

 sented their appearance in the mandibles in his Fig. 2, Plate I. 



The large tubes in the chitin alter in shape according to their 

 position. Those in the middle of the sensory surface of the flabellum, 

 in their course through the chitinous layers, are hardly bent at all ; 

 as they approach the two lateral edges of this surface, their long thin 

 neck becomes bent more and more, the bending always being directed 

 towards the middle of the surface (see Fig. 146) ; in this way the 

 chitinous tubules increase more or less regularly in length from 

 the centre of the organ to the periphery. The large basal part of 

 the conical tube contains, besides the chitinous tubule, a number 

 of nuclei which are confined to this part of the tube ; some of these 

 nuclei look like those belonging to nerve-fibres, others are apparently 

 the nuclei of the chitinogenous membrane lining the tube. I have 

 never seen any sign of nerve-cells in the tube itself. 



The only other kind of sense-organ I have found in connection 

 with these sense-organs are a few spike-like projections, the appear- 

 ance of which is given in Fig. 149. I have always seen these in the 

 position given in Fig. 146 (sp.), i.e. at the junction of the surface 

 which contains the sense-organs and the surface which is free from 

 them. They are, so far as I have seen, not very numerous ; I have 



