SENSE ORGANS. 139 



paired or unpaired swellings (Trigla, Orthagoriscus) at the origin of 

 the spinal nerves. 



The eyes are seldom hidden beneath the skin and the muscles 

 (Myxine, Petromyzon, AmUyopsis). In Amphioxus they are repre- 

 sented by a pigment spot lying directly on the central nervous 

 system. In all other fishes they are characterised by possessing 

 a flat cornea and a large, almost spherical crystalline lens, the 

 anterior surface of which projects far out of the pupil (fig. 589). As 

 peculiar structures of the eyes of fishes are further to be mentioned 

 the so-called choroideal gland a vascular body (rete mirabile) usually 

 projecting at the entrance of the optic nerve, as well as a fold of 

 the choroid known as the processus falciformis, which traverses the 

 retina, and the campanula Halleri which is attached to the lens. 



The auditory organ * (absent only in "Amphioxus) consists only of 

 the labyrinth (fig. 578, /), and in Teleosteans, 

 Ganoids, and Chimcera lies partly in the cranial 

 cavity, surrounded by fatty tissue. It is worthy -P 

 of notice that in Cyprinoidce Characince, Siluridce, 

 and others, the labyrinth is connected with the 

 swimming bladder by a chain of small bones. 



The olfactory organ in Amphioxus consists of 

 a simple unsymmetrical pit at the anterior end 

 of the nervous centre. In Cyclostomes also it FIG. 589. Horizontal 



. . section through the 



consists of a simple tube, with an unpaired median e ye of Etox 



opening. All other fishes possess double, and Co ' comea ; z lenfi ; 



*? r ... -P/. processus falci- 



indeed with the exception of the Dipnoi blindly- formis; CH, cam- 

 closed nasal cavities, the internal surface of which 



is considerably increased by folds of the mucous ossifications of the 



-, sclerotic. 



membrane. 



The sense of taste seems to be less developed. It is located in 

 the buccal cavity, and especially in the richly innervated part of the 

 soft palate. For the tactile sense, lips and their appendages the 

 frequently appearing barbules probably serve. Certain isolated 

 rays of the ventral fin may also, on account of their rich nerve 

 supply, be regarded as tactile organs (Trigla}. The nervous organs 

 of the so-called mucous canals, which we have before mentioned, 

 constitute an organ of a special sense. 



* Compare E. H. Weber, " De aure et auditu hominis et animalium." P. I., 

 " De aure animalium aquatilium." Lipsiae, 1820. 



C. Hasse, " Anatomische Studien." Heft 3 : " Das Gehororgan der Fische." 

 Leipzig, 1872. 



