548 



STRUCTURE OF VERTEBRATA 



into the mouth, and serve for the entrance of air. The 

 pecuhar 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 

 {Serraftus, salmon) and Amphibians a common ectodermic 



thickening seems to 



T ^^ -^^ form the rudiment 



^ ^ I from which the ear, 



the lateral line, and 

 a pre-auditory sen- 

 sory patch are de- 

 rived. The auditory 

 sac sinks farther in, 

 and the originally 

 wide opening to the 

 exterior becomes a 

 long narrow tube. 

 In Elasmobranchs, 

 which exhibit many 

 primitive features, 

 this condition is us- 

 ually retained in the 

 adult ; in other Ver- 

 tebrates the tube loses 

 its connection with 

 the exterior, and be- 

 comes 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 



Fig. 310.— Diagram showing development 

 of ear in a Vertebrate (cartilaginous 

 fish). 



BR., Brain ; /., opening of the ectodermic insinking 

 that forms the inner ear ; E. C, ear cavity at its 

 simplest ; P., particles of lime ; B.V., blood 

 vessels. 



