386 STRUCTURE OF VERTEBRATES. 



as the brain is itself an involution of epiblast, the eye may 

 be also referred to external nerve-cells. 



{a) Branchial Sense-Organs. — In many Fishes and Amphib- 

 ians there are lateral sense-organs which form the " lateral 

 lines," while the more anterior and more primitive are con- 

 nected with gill-clefts, or in all likelihood were connected 

 with gill-clefts which have disappeared. In Sauropsida and 

 Mammals these branchial sense-organs are no longer distinct 

 as such. 



{b) The Nose. — It is possible that the sensory pits of skin 

 which form the nasal sacs are two branchial sense-organs. 

 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 hagfish and lamprey is referred to in 

 the chapter on Cyclostomata, pp. 404, 406. 



(c) The Ear. — From an involution of epiblast the ear also 

 develops, and perhaps it also is a branchial sense-organ. The 

 narrowed neck of the involution, originally opening externally, 

 persists in this primitive state in some Elasmobranchs, but 

 usually ends in the dura mater of the skull. It is called the 

 aqueductus vestibuli, or the ductus endolymphaticus. 



The cavity or vestibule of the ear is divided into a larger 

 utriculus, and a smaller sacculus. Round the utriculus three 

 semi-circular canals are formed, except in the lamprey which 

 has only two, and in the hagfish which has only one. 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 in Sauropsida 

 and Mammals. 



The whole cavity of the ear is called the membranous 

 labyrinth, and contains a fluid called endolymph, but it 

 becomes ensheathed in mesoblast, which forms a cartilaginous 

 or bony labyrinth. Between the membranous labyrinth and 

 Its external sheath there is a fluid called perilymph. 



Within the ear are various patches of sensory epithelium 

 with branches of the auditory nerve. There are calcareous 

 concretions (otoliths) near the sensory patches, except in the 

 cochlea of Mammals. 



