224* PISCES. 



slit, we find what is called the falciform process ; this is a fold of the 

 choroid invested by black pigment, that passes through the above- 

 mentioned slit and the substance of the vitreous body, and is in- 

 serted into the side of the capsule of the lens. A filament of the 

 ciliary nerve frequently accompanies it, and dilates into a pear-shaped 

 knot forming the pyriform body or campanula Halleri. Occasionally, 

 as in Muraena Conger, two falciform processes have been observed, 

 an anterior and a posterior one, which hold the lens between them 

 like two poles. These structures, such as the falciform process, 

 which reminds us of the pecteri in the eye of Birds, are wanting in 

 the Plagiostomi and many Malacopterygii, e. g. the Carp. 



The bulb of the eye is but slightly moveable in Fishes. In the 

 Osseous Fishes it is attached to the orbit by a short ligament near 

 to the insertion of the optic nerve ; in the Plagiostomi it stands upon 

 a moveable cartilaginous stalk, which articulates with a short pro- 

 cess of the sclerotic. Four straight and two oblique muscles move 

 the eye, and between them we find some adipose and cellular tissue. 

 In the Bdellostoma the rudimentary eyes are entirely destitute of 

 muscles, which in Petromyzon among the Cyclostomi are present in 

 their usual number and position. 



Organs of Hearing. 



IN the structure of the organs of hearing in the class of Fishes we 

 meet with an interesting series of gradations, ranging from the low- 

 est and simplest stage, wherein their structure resembles that of the 

 Invertebrate animals, to that which exhibits a combination of parts 

 corresponding to some one or other of the more highly organized 

 Vertebrata. 



The general arrangement of the auditory organs in the Osseous 

 Fishes is as follows : the whole membranous labyrinth lies for the 

 most part free within the cranial cavity, and adjacent to the brain, 

 or it is only imperfectly and partially enclosed in bones, which have 

 been already (p. 189) regarded as belonging to the system of tem- 

 poral and occipital bones. These cavities are, as it were, lateral 

 continuations of the cranial cavity, and are, like the latter, often filled 

 up by a lax cellular and adipose tissue containing freely floating 

 drops of oil. In cases where the auditory ossicles (which we shall 

 presently describe) of the membranous labyrinth are of very large 

 size, the cavities for containing them are dilated into a bladder-like 

 form, as in Gadus, Sciaena, and in a lesser degree in Perca and 



