81 



This nerve (n, Fig 6) runs posteriorly along the surface of the scale, beneath the 

 epithelium of the dermal tube, to the sense-organ. The sense-organ consists of a 

 number of thin elongated cells placed so that their length is almost perpendicular to the 

 wall of the dermal tube. Each of these cells contains in its lower portion a spindle- 

 shaped swelling which contains a nucleus. One end of each cell reaches the surface of the 

 sense-organ, and the other reaches its basement membrane : the cells are all in contact, 

 the nuclear swellings being so arranged that the cells are packed in the least possible 

 compass. Thus the nuclear swelling of one cell is at its base, that of the next some- 

 what higher up, so that the nuclei form two rows, and the nuclei of some of the cells 

 are even higher still, forming a third row. But it is only the nuclei which are in two 

 or three layers, the cells are all in a single layer side by side. The nuclei, in spite of 

 the arrangement described, occupy more space than the cells, and consequently the 

 base of the sense-organ is broader than its surface, so that only the central cells are 

 straight, the external cells curving at their upper ends towards the centre of the sense- 

 or^an. The upper end of each cell ends in a delicate protoplasmic hair which projects 

 into the cavity of the dermal tube. Tiie lower end of each cell is continuous with one 

 of the ultimate fibrils of the nerve previously mentioned. I have not made out this 

 connection in all the cells, but I have traced it in some, and believe that it exists in all. 

 In some preparations a kind of clot is seen on the surface of the sense organ, and 

 sometimes this seems to have separated from the surface of the organ, breaking off the 

 sensory hairs and taking them with it. This clot often present a laminated appearance, 

 as if formed of thin layers one over another. It has been seen in some fishes in an 

 organ examined in a perfectly fresh condition. Prof. Emery''* concludes that the laminse 

 of the clot, which is generally called the cupula, are successive cuticles secreted by 

 the peripheral cells of the sense organ. I think it is difiicult to accept this conclusion, 

 and am inclined to think the cupula is, during life, of a mucous nature, and therefore 

 semi-liquid. It seems certain that the sensory hairs are imbedded in the cupula. It 

 is difficult to understand how such cells as those of the sense-organ should secrete 

 mucus or form a cuticle : perhaps the cupula is nothing more than the ordinary mucus 

 of the dermal tube which keeps a constant position in preparations because it is retained 

 by the numerous sensory hairs. 



There is not a sense-organ to everj' scale of the lateral line ; in the middle of the 

 body there is a sense organ on every third scale, that is to say, there are two scales 

 bearing no sense-organs between two scales which bear them. The position of the 

 sense-organ in relation to the scale on which it is situated is always the same. 



The function of these sense organs is still entirely unknown. It has been suggested 

 that they convey to the fish a sense of its position in the water, and so enable it to 

 retain its vertical position, but the evidence for this is not very conclu?ive : if it were 

 true we should expect to find the organs atrophied in the flat fishes which have 



* " Fauna und Flora des Golfes von Xeapel," "Fierasfer." 



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