410 CHORDATA. 
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 77s, 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-mmuscles 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 /ateral-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 agueductus vestibul. 
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 ampulle, to which the 
8th nerve gives off numerous branches. The sac itself is 
now known as the vestibule. In many fishes, eg., the skate, 
its cavity remains connected with the exterior by the ague- 
ductus vestibuli, In the skate this zzmer ear (or membranous 
abyrinth) lies close to the hyomandibular cartilage, near 
which is the spiracle. Vibrations of the water may be trans- 
mitted through the hyomandibular to the inner ear. 
