202 LECTURE VIII. 



with special muscles ; these processes are supported by peculiar 

 cartilages more or less intimately connected with the proper olfactory 

 cartilaginous sacs, and representing the superadded cartilages of 

 the 'alse nasi' in higher Vertebrata.* They have their proper 

 muscles ; whence we must conclude that these Fishes scent as well 

 as smell : i. e. actively search for odoriferous impressions by rapidly 

 changing the current of water through the olfactory sac. 



The Organ of Sight makes its appearance in the lowest of Fishes, 

 e. g. the Lancelet and Myxine, under as simple a form as in the 

 Leech : a minute tegumentary follicle is coated by dark pigment, 

 which receives the end of a sj^ecial cerebral nerve. This simple eye- 

 speck, the first mechanism for the appreciation of light, is repeated 

 in the Ambhjopsis spelceus {fig. 50. 2), Eudimental eyeballs covered 

 by the skin exist in the Aioterichthys ccecus : the small, but more 

 complex eyes of the Lepidosiren, with crystalline and vitreous 

 humours, choroid and sclerotic tunics, are also covered by the skin ; 

 but this becomes transparent whei'e it passes over them, and, ad- 

 hering to the sclerotic, forms a ' cornea.' The eyes of the Eel-tribe 

 and the Siluroid Fishes are small : they are of moderate size in the 

 Plagiostomes and Ganoids ; but in most Osseous Fishes the eyes are 

 remarkable for their large size, Avhich becomes enormous in some, 

 6. g. Orthagoriscus (Prep. 1665. a), Myripristis, Priacanthus. The 

 eyes are usually placed in orbital cavities, one on each side of the 

 head ; only in the unsymmetrical Flat-fish are they both placed on 

 the same side : in the Star-gazer ( Uranoscopus) the eyes are ap- 

 proximated on the upper surface of a nearly cubical head, and are 

 directed towards the heavens : in the Hammer-headed Sharks they 

 are supported on long outwai'd projecting pedicles. 



The optic nerve (unfolded in. fig. 57. a) usually perforates the eye- 

 ball obliquely out of its axis ; but sometimes directly in its axis. In 

 Osseous Fishes it is compressed where it passes through the sclerotic 

 and choroid, and then forms the retina by unfolding itself like a fan 

 spread out and bent into the form of a cone, leaving a fissure (b) 

 where the free lateral borders meet after lining about two-thirds of 

 the hollow globe. This fissure extends, of course, from the entry 

 of the nerve to the anterior margin of the retina, and through it 

 a fold of the innermost layer of the choroid extends into the vitreous 

 humour, sometimes accompanied by the dark pigmental Ruyschian 

 layer, as is shown in the preparation of the eye of the Bonito 

 (No. 1651.). The fold of the vascular choroid, whether accompanied 



* See the description of these ' nasenfliigelknorpel ' in xxi. p. 171. 



