220 Mr Heathcote, Note on a peculiar sense organ. [Nov. 24, 1884. 



formed by the folds, is closely lined by a number of chitinous hairs. 

 Each hair consists of a short and thick elliptical basal portion, one 

 end of which is prolonged into a long fine hair which projects into 

 the sac while the other end terminates in a short stump-like point 

 which projects internally through the chitinous lining of the sac. 

 The chitin of the sac is not of uniform thickness. In the recesses 

 and on the folds which bound them it is smooth, but in the 

 pouches, where the hairs exist, it is thrown into a series of ridges 

 and depressions which give it a reticulated appearance. The base 

 of a hair projects into each depression. 



Internally the hypodermic cells which form the ordinary matrix 

 of the exoskeleton line the chitin as far as the folds bounding the 

 lateral recesses. Here they change their character and gradually 

 become continuous with a thick layer of sensory epithelium which 

 lines the internal surface of the pouches. At the folds which bound 

 the median dorsal recess the sensory epithelium becomes continuous 

 with hypodermic cells which line the folds, but at the top of the 

 median recess there is a mass of ganglionic cells which differ from 

 those composing the sensory epithelium. The hypodermic cells 

 are large, rather columnar, and have a well-defined nucleus. The 

 cells of the sensory epithelium are elongated and at the end next 

 the chitin are prolonged into a thin projection of less diameter 

 than the rest of the cell. The prolongation of each cell is just 

 beneath the base of a chitinous hair, and I believe that the extreme 

 point of each cell prolongation fits into a cavity in the base of a 

 hair. The cells at the top of the median recess are elongated and 

 in some cases have their internal end bifurcated. They are larger 

 than the sense cells. The nerve supply is furnished by two short 

 thick nerves which arise from the front part of the sub-oesophageal 

 ganglion and pass to the posterior region of the organ, where thev 

 break up into fibres which supply the two divisions of the organ. 

 This double nerve supply and the partially divided shape of the 

 sac, already mentioned, prove in my opinion conclusively that the 

 organ is a double one. 



I believe this organ to be homologous with the tympanic organ 

 of Insects. I regard the chitinous lining of each pouch as equiva- 

 lent to the tympanic membrane of the insect auditory organ. In 

 both cases we find a nerve breaking up into fibres which are con- 

 nected with terminal structures which terminate in depressions in 

 the chitin. V. Hensen regards the terminal structures or auditory 

 rods (Horstifte) in the insect organ as homologous with the hairs 

 in the crustacean auditory sac. If his arguments hold good I think 

 the chitinous hairs of the organ I have just described may be 

 compared to both the structures just mentioned. From all that I 

 have said it follows that I regard the organ of Scutigera as belong- 

 ing to the class of organs usually described as auditory. 



