SENSE ORGANS. 493 



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 

 (Serranus, 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. 



