SENSE ORGANS. 475 



They are lined by epithelium in great part sensory, and are 

 connected posteriorly with the olfactory nerves. In all 

 Fishes, except Dipnoi, the nasal sacs remain blind ; in 

 Amphibians, and in all the higher Vertebrates, they open 

 posteriorly into the cavity of 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 further in, and the 

 originally wide opening to the exterior becomes a long 

 narrow tube. In Elasmobranchs, which exhibit many 

 primitive features, this condition is 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. 



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 important 

 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. 



The parts which we have so far considered constitute together the 

 membranous labyrinth of the ear. Round about them the mesoblast 

 (mesenchyme) forms a two-layered envelope. Its inner layer disin- 



