112 FISHES. 



sufficient evidence that the vision, at least of certain species 

 is by no means devoid of clearness and precision. 



The eye of Branchiostoma is of the most rudimentary con- 

 dition. It is simply a minute speck coated by dark pigment, 

 and receiving the end of a short nerve. In Myxinoids the 

 minute rudiment of the eye is covered by the skin and mus- 

 cles. This is also the case in many of the blind Teleosteous 

 fishes; however, whilst in the former fishes the organ of sight 

 has not attained to any degree of development, the rudiment- 

 ary eye of blind Teleostei is a retrogressive formation, in which 

 often a lense and other portions of the eye can be recognised. 

 In fishes with a well-developed eye it is imbedded in a layer 

 of gelatinous and adipose substance, which covers the cavity of 

 the orbit. A lacrymal gland is absent. In the orbit of one fish 

 only, Chorismodentex, an organ has been found which can be 

 compared to a saccus lacrymalis. It is a round, blind, wide 

 sac, of the size of a pea, situated below the anterior corner of 

 the orbit, between the maxillary bone and the muscles of the 

 cheek, communicating by a rather wide foramen with the 

 orbital cavity. The membrane by which it is formed is con- 

 tinuous with that coating the orbita. In the Chondroptery- 

 gians the eyeball is supported by and moves on a cartilaginous 

 peduncle of the orbital wall. In the majority of Teleosteans, 

 and in Acipenser, a fibrous ligament attaches the sclerotic to 

 the wall of the orbit. The proper muscles of the eyeball 

 exist in all fishes, and consist of the four Musculi recti and the 

 two M. oUiqui. In many Teleostei the former rise from a 

 subcranial canal, the origin of the M. rectus externus being 

 prolonged farthest backwards. The JRecti muscles are extra- 

 ordinarily long in the Hammerheaded Sharks, in which they 

 extend from the basis cranii along the lateral prolongations 

 of the head to the eyes, which are situated at the extremities 

 of the hammer. 



In all fishes the general integument of the head passes 



