SENSE ORGANS. 493 
Pd 
anterior nares to mouth are arched over and open 
posteriorly into the front of the mouth. In Amphibians, 
and in all the higher Vertebrates, the nasal chambers open 
posteriorly into the mouth, and serve for the entrance of 
air. The peculiar nostril of hag-fish and lamprey is referred 
to in the chapter on Cyclostomata. 
The ear in Invertebrates develops as a simple invagina- 
tion of the ectoderm, forming a little sac, which may become 
entirely detached from the epidermis, or may retain its 
primitive connection; so in Vertebrates, at an early stage, 
an insinking forms the auditory pit. In some. Fishes 
(Servanus, salmon) and Amphibians a common ectodermic 
thickening seems to form the rudiment from which the ear, 
the lateral line, and a pre-auditory sensory patch are 
derived. The auditory sac sinks farther in, and the ori- 
ginally wide opening to the exterior becomes a long narrow 
tube. In Elasmobranchs, which exhibit many primitive 
features, this condition is usually retained in the adult; in 
other Vertebrates the tube loses its. connection with the 
exterior, and becomes a blind prolongation of the inner 
ear—the aqueductus vestibuli, or ductus endolymphaticus. 
In Anura the ductus endolymphaticus gives rise to a long 
sac dorsal to the spinal cord giving off outgrowths in which 
the “ calcareous bodies” lie. 
The auditory vesicle, at first merely a simple sac, soon 
becomes very complicated. It divides into two chambers, 
the larger utriculus and the smaller sacculus. From the 
utriculus three semicircular canals are given off, except in 
the lamprey and hag, which have two and one respectively. 
From the sacculus an outgrowth called the cochlea or 
lagena originates ; it is little more than a small hollow knob 
in Fishes and Amphibians, but becomes large and im- 
portant in Sauropsida and Mammals. 
As this differentiation of the parts of the internal ear takes place, the 
lining epithelium also becomes differentiated into flattened covering cells 
and sensory auditory cells. The auditory cells are arranged in patches 
to which branches of the auditory nerve are distributed. With these 
sensory patches calcareous concretions (otoliths) are associated, except 
in the cochlea of Mammals. 
The fact that lime salts are often deposited in the skin, and that the 
ear-sac arises as an insinking of epiblast, may perhaps shed some light 
on the origin of otoliths. 
