Ixxviii Introduction. 



trig^emimis, and upon the trunk along- the lateral line. The ol- 

 factory organ is an azygos sac in the Marsipohranchii , hence called 

 ' Monorrhina;' it ends blindly in the sub-order Tetrowyzoniulae, 

 which are hence called ' Hyperoartii,' but opens into the pharynx 

 in the Myxinoids, hence called ' Hyperotreti.' All hig"her Verte- 

 brata have paired nasal sacs^ and have consequently been styled 

 ' Amphirrhina.' In the Dipnoi, the nasal sacs have valvular poste- 

 rior openings, externally to the pterygo-palatine teeth ; in all other 

 amphirrhine Fishes they end blindly. The eyes are relatively large 

 in Fish, the cornea is flat, the lens spheroidal. The Elasmobranchii 

 possess eyelids, and sometimes a membrana nictitans. A peculiarly 

 piscine rete mirabile, the ' choroid gland,' exists in most Fishes, 

 except the MasmobrmicJdi and Ganoidel, between the layers of the 

 choroid, where it suri'ounds the entrance of the optic nerve, A 

 structure homologous with the * pecten ' of Birds exists in many 

 Teleostei, where it is known as the ' processus falciformis.' Certain 

 structures, consisting of pigment specks with lens-like bodies inlaid 

 in their substance, have been found regularly arranged between the 

 branchio-stegal rays, upon the head, and in two pairs of longitu- 

 dinal rows on the ventral surface of Chauliodes and Stoniias, and 

 have been regarded as accessory eyes. 



The auditory apparatus of the Marsipohranchii consists of a 

 vestibule, with, in Myxinoids one, and in Pelromyzontidae two 

 semicircular canals, contained in cartilaginous capsules attached to 

 the skull laterally. In all higher Fishes there are three semicircular 

 canals. In the Sharks and Rays, and in the Dipnoi^ the mem- 

 branous labyrinth is entirely surrounded by the cranial walls, but 

 in the Cliimaerae, the Ganoidei, and the Teleostei, a median portion 

 is always to be found lying free in the cranial cavity. The air- 

 bladder of some Acanthopteri, and of many Physostomous Teleostei, 

 comes into relation with the membranous labyrinth, either directly 

 or through the intermediation of ossicula. In ElasmobrancJdi 

 canals exist, marking the line along which the integument was 

 invaginated, to form the 'internal ear.' The electric organs of 

 Fishes are not represented in higher Vertebrata. They are most 

 largely developed in the Torpedo and Gymnotus, and in a lower 

 degree in Malapterurus and Mormyrus. They are found in various 

 parts of the body, but consist essentially of prismatic columns, 

 made up by the superposition of flat plates, to which nerves from 



