426 FISHES 



of an inner part shaped like a rounded sac, and an outer part 

 opening by a wide oblique aperture on to the surface of the 

 skin : the two parts are separated by a constriction. Both 

 parts are surrounded internally by, first, a layer of iridocytes 

 forming the reflector, and, outside this, by a layer of pigment. 

 The inner sac is filled with gland tubes, radiating from the 

 mouth of the sac to the walls. The tubes are lined by large 

 granular cells, which seem to pour their secretion into the 

 cavity of the sac, especially towards the mouth. The contents 

 of the outer part of the organ are quite different : here there 

 are a number of short cylindrical or prismatic columns perpen- 

 dicular to the surface of the skin. Each of these columns con- 

 sists of a core of blood-vessels and nerves, surrounded by 

 slender radiating cells some of which are enlarged at their 

 outer ends, and contain a transparent refringent body. These 

 cells have been supposed to be the special phosphorescent cells, 

 but if the light is produced by the glandular cells of the inner 

 sac, it would seem more probable that the cells of the outer 

 portion are merely refracting structures. In the ventral organs 

 of Sternoptyx which are to a great extent united internally, al- 

 though the apertures of those of the two sides are separated by 

 a median ridge, the external part does not contain columns 

 like those above described, but appears to contain glandular 

 tubes. It is evident that thorough investigation of fresh speci- 

 mens of the fish is still required in order to ascertain with 

 more certainty the functions of the different parts of the organs. 

 The two ventral rows of organs in Astronesthes are of two 

 kinds, one larger, compound in structure, the other smaller and 

 simple. The compound organs are somewhat similar to those 

 above described in Sternoptyx; they consist of an internal 

 spherical part separated by a constricted neck from an outer 

 part shaped like a paraboloid cup with its aperture towards the 

 surface of the skin. The spherical portion is divided by radiat- 

 ing lines into tubes containing glandular cells ; the neck portion 

 consists of two transverse layers of large cells ; the outer cup is 

 filled with a somewhat granular tissue in which radiating lines 

 are distinctly visible ; these lines are fibres radiating from the 

 neck of the organ to its aperture ; each fibre is surrounded by 

 slender cells arranged like the hairs on the tail of a squirrel. It 

 is possible that this tissue of the cup is refractive in function, 



