THE TELEOSTEI. 165 



Each of the nasal sacs usually opens extei-nally by two 

 apertures. In some Gymnodonts a solid tentacle is said to 

 take the place of a nasal sac. 



The eyes are abortive in the Blind-fish of the caves of 

 Kentucky [Ambhjopsis speheus). A fibrous band often 

 passes from the back of the orbit to the sclerotic, and re- 

 presents the cartilaginous pedicle of the Elasmobranchs. 

 There is no nictitating membrane, but immoveable external 

 eyelids may be developed. The choroidal gland, mentioned 

 above, surrounds the optic nei-ve between the sclerotic 

 and the choroid. Yery generally, a falciform process of the 

 latter membrane traverses the retina and vitreous humour t( > 

 the crystalline lens. This represents the pecten of higher 

 Vertebrata. As in other fishes, the lens is spheroidal, and 

 the cornea flat. The sacculus of the auditory organ con- 

 tains large solid otoliths, which are usually two in number 

 — the larger, anterior one, is termed Sagitta ; the smaller. 

 posterior, Asteriscus. There are always three large semi- 

 cii'cular canals. 



The reproductive organs are either solid glands which 

 burst into the abdominal cavity, whence their reproductive 

 elements are conveyed away by abdominal pores ; or, as is 

 more usual, they are hollow organs, and are continued back- 

 wards into ducts which open beside, or behind, the urinary 

 aperture. 



Some few Teleostei are ovoviviparous (e.g., Zoarces vivi- 

 pcnnis), the eggs being retained in the interior of the ovary, 

 and hatched there. In the male Syngnathus, and other 

 Lopliohrancliii, integumentary folds of the abdomen grow 

 down and form a pouch, into which the eggs are received, 

 and in which they remain until they are hatched. 



The young of osseous fishes are not known to undergo 

 any metamorphosis, nor are they provided with external 

 gills, nor with spiracula. 



The classification of the Teleostei is not yet in a thoroughly 

 satisfactory state, and the following arrangement must be 

 regarded as provisional : — 



