420 ACCESSORY EYE W IX THE VE RTF BRAT A. 



however be inclined to suggest that the eye had in this case ceased for a 

 period to be employed ; and that it has been re-developed again in some 

 of the larval forms. Its characters in the Tuiiicata are by no means 

 constant. 



Accessory eyes in the Vertebrate. 



In addition to the paired eyes of the Vertebrata certain organs are 

 found in the skin of a few Teleostei living in very deep water, which, though 

 clearly not organs of true vision, yet present characters which indicate 

 that they may be used in tlie perception of light. The most important of 

 such organs are those found in Chauliodus, Stomias, etc., the significance 

 of which was first pointed out by Leuckart, while the details of their 

 structure have been recently worked out by Leydig 1 and Ussow. They 

 are distributed not only in the skin, but are also present in the mouth and 

 respiratory cavity, a fact which appears to indicate that their main function 

 must be something else than the perception of light. It has been suggested 

 that they have the function of producing phosphorescence. 



Another organ, piobably of the same nature, is found on the head of 

 Scopelus. 



The organs in Chauliodus are spherical or nearly spherical bodies in- 

 vested in a special tunic. The larger of them, which alone can have any 

 relation to vision, are covered with pigment except on their outer surface. 

 The interior is tilled with two masses, named by Leuckart the lens and 

 vitreous humour. According to Leydig each of them is cellular and 

 receives a nerve, the ultimate destination of which has not however been 

 made out. According to Ussow the anterior mass is structureless, but 

 serves to support a lens, placed in the centre of the eye, and formed of a 

 series of crystalline cones prolonged into fibres, which in the posterior part 

 of the eye diverge and terminate by uniting with the processes of multipolar 

 cells, placed near the pigmented sheath. These cells, together with the 

 fibres of the crystalline cones which pass to them, are held by Ussow to 

 constitute a retina. 



Eye of the Mollusca. 



(36-2) N. Bobretzky. "Observations on the development of the Cephalopoda" 

 (Russian). Nachrichten d. kaiserlichen Gesell. d. Freunde der Naturwiss. Anthropolog. 

 Ethnogr. bei d. Universitdt Moskau. 



(363) H. Grenadier. " Zur Eutwicklimgsgeschiclite d. Cephalopoden." Zeit. f. 

 wiss. Zool., Bd. xxiv. 1874. 



(364) V. Hensen. "Ueberd. Auge eiuiger Cephalopoden." Zeit. f. wiss. Zool., 

 Vol. xv. 1865. 



(365) E. E. Lanke ster. " Observations on the development of the Cephalopoda." 

 Quart. J. of Micr. Science, Vol. xv. 1875. 



(366) C. Semper. Ueber Sehnniane von Typu* d. irirbelthierauf/en. Wiesbaden, 

 1877. 



1 F. Leydig. "Ueber Nebenaugen d. Chauliodus Sloani." Archiv f. Anat. itnd 

 Phys., 1879. M. Ussow. " Ueb. d. Bau d. augena'hnlichen Flfcken einiger Knochen- 

 fische." Sul. d. la Soc. d. Naturalistes de Moscou, Vol. LIV. 1879. Vide for general 

 description and further literature, (lunther, The Study of Fishes, Edinburgh, 1880. 



