4IO CKORDATA. 



layer, a peculiarity of the vertebrate eye. If it be recollected 

 that the brain is invaginated from the dorsal epiblast and 

 the eye is an invaginated part of the brain, it will be clear 

 that the rods and cones really lie on the morphological outer 

 surface, the normal situation for sensory elements. 



The third element of the eye is mesoblastic ; it consists 

 of a choroid coat carrying blood-vessels and partially cover- 

 ing the lens as the /m, and the sclerotic^ a hard cartilaginous 

 capsule enveloping the eye. In front of the lens it is trans- 

 parent and forms the cornea^ the anterior chamber being 

 formed between it and the lens. 



To this we must add the eye-muscles which are inserted 

 in the sclerotic and serve to move the eye. They have been 

 noticed in the skate and do not differ essentially in any 

 type. 



Obliquus superior innervated by 4th nerve. 



Obliquus inferior innervated by 3rd nerve. 



Rectus superior innervated by 3rd nerve. 



Rectus inferior innervated by 3rd nerve. 



Rectus internus innervated by 3rd nerve. 



Rectus externus innervated by 6th nerve. 

 Accessory organs, such as eyelids and lacrymal glands, are 

 added in terrestrial types. 



The third sense-organs, or auditory sacs, appear to be a 

 single much hypertrophied pair of lateral-line sense-organs, 

 organs which were noticed in the skate but are not found as 

 such in terrestrial Vertebrata. The auditory sacs arise as 

 paired pits of the epiblast, far back on the head. Each pit 

 swells out as an auditory sac, its connection with the epiblast 

 becoming constricted into a thin duct, the aqueductus vestibuli. 

 The walls of the sac then grow out into three (one in 

 Myxine) semi-circular canals, long tubes which run in a semi- 

 circle in three separate planes and open at each end into the 

 sac. Their bases are swollen into ampullae, to which the 

 8th nerve gives off numerous branches. The sac itself is 

 now known as the vestibule. In many fishes, e.g., the skate, 

 its cavity remains connected with the exterior by the aque- 

 ductus vestibuli. In the skate this inner ear (or membranous 

 abyrinth) lies close to the hyomandibular cartilage, near 

 which is the spiracle. Vibrations of the water may be trans- 

 Id through the hyomandibular to the inner ear. 



