ZOOLOGY. 475 



likewise developed on the head, and eveu on the brauchiostegal rays. 

 Nearly quarter of a century ago the illustrious German histologist 

 KoUiker had suggested that these organs were " essentially nervous, 

 and present the nearest resemblance to the electrical organs of fishes." 

 But this suggestion received no notice, and has only recently been res- 

 urrected by Professor Bell. In 1865 Professor Leuckart published the 

 results of a special examination, and expressed the belief that they 

 were accessory eyes. In 1879 Professor Ussow examined the organs 

 anew in seven distinct generic types, and urged that they belonged to 

 two different categories, those of some fishes being accessory eyes, 

 while those of others were special glandular organs. The two kinds, 

 it was claimed, were never developed in the same fish. Finally, in 1881, 

 Professor Leydig attacked the problem and made known the results of 

 his examinations in a special work (illustrated by ten plates), of which 

 an abstract has been given by Prof. F. Jeffrey Bell, from which, in 

 the absence of the original, we derive the conclusions of the Bonn pro- 

 fessor. 



The peculiar organs of the fishes under consideration are referable, 

 according to Leydig, to three categories: (1) "Eye-like organs," (2) 

 " mother-of-pearl-like organs," and (3) " luminous organs," but the last 

 two are confined to the scopeli. 



The ey^-like organs are " saccular in form and divisible into a bulb, 

 a neck, and an orifice, and this orifice is always directed downward." 

 They have " an investment of brown pigment, a layer with a metallic 

 glitter, a gray inner body, and a surrounding lymphatic space. The 

 investment is derived from the general integument of the body, and ihe 

 pigment granules are contained in the cells of the underlying connect- 

 ive tissue ; the metallic layer consists of iridescent plates, rods, or 

 fibres. The gray inner body is divisible into two portions, the hinder 

 and larger of which fills the sac, while the anterior and smaller occu- 

 pies the narrower neck. The striate appearance of this part " is ap- 

 parently due " to the presence of a framework of connective tissue, which 

 sends rays into or forms a network in it; into this gray part there 

 further proceeds a nerve, the fibers of which j^robably come into con- 

 nection with the contained cells." These organs were regarded by 

 Ussow as having one or other of two functions, but according to Leydig 

 there is no essential difference between them, and both must have the 

 same function — whatever that may be. 



The mother-of-pearl organs "have in all cases an outer brown invest- 

 ment, a metallic layer, a gelatinous internal body formed of connective 

 tissue ; they are provided with nerves and blood-vessels, and are sur- 

 rounded by a lymphatic space. The metallic plates are regarded by 

 Ussow as special cells, but Leydig looks upon them as altogether simi- 

 lar to the minute iridescent bodies found on the skin. The gelatinous 

 portion is made up of delicate radiate cells, which give rise to a net- 

 work, and an intermediate soft substance?'. 



