338 



Vertehrata. 



consists of a saccular chamberj and three semi-circular canals; 

 tlie latter are tubes opening at both ends into the vesicle, to which 

 they are attached like hollow handles. The sac itself is divided by a 

 constriction into two portions, the sacculus and the utriculus ; the 

 canals open into the latter, and each possesses a swelling at one end 

 (ampulla). The sacculus usually bears an evagination {ductus 

 coehlearis), which is^ in Pish, Amphibians, and many Reptiles, a 

 short minute pouch; in some Reptiles (the Crocodiles) and in the 

 Birds, a longer tube ; but attains its highest development in the 

 Mammalia, where it forms a long spiral canal. This whole vesicle, 

 which consists of epithelium, surrounded by a thin layer of connective 



tissue, is termed the membranous 

 labyrinth. This is the essential 

 auditory organ which occurs in all 

 Vertebrata, Amphioxus alone ex- 

 cepted, and usually includes all 

 the parts named. There are certain 

 cells in the epithelium provided 

 with one or more processes, to 

 which the branches of the auditory 

 nerve are distributed, and which 

 are closely connected with the sense 

 of hearing. In the labyrinth there 

 are otoliths, either small 

 crystals or larger calcareous bodies 

 (Teleostei). The membranous laby- 

 rinth, which is inclosed in the 

 lateral wall of the skull, constitutes in Fish the whole auditory 

 apparatus ; in other Vertebrata various accessory structures 

 are usually connected with it (tympanic-cavity. Eustachian tubes, 

 tympanum, ear-bones), which will be considered in the different 

 groups. 



Those portions of the skull which closely surround the membranous labyrinth 

 are often {e.g., in the Mammaha) more compact in structure than the rest of the 

 bone, and may be entirely separated, giving exactly the form of the enclosed 

 membranous labyrinth : this is termed the bony labyrinth. 



The alimentary canal is divisible into the following parts : 

 buccal cavity, oesophagus, stomach, small intestine, and rectum.' Of 

 the structures connected with the spacious buc c al-cavity, the 

 teeth will be considered first. 



The teeth, both in structure and development, are essentially 

 characteristic of the Vertebrata. They are present not only in the 

 mouth, but in many Fish (especially in the Selachians) on the skin 

 also. They occur within the mouth in all classes (with the exception 

 of the Lancelot, Amphioxus), although they are frequently absent. 

 In the simplest case (Fig. 279 A) the formation of teeth occurs 



Pig. 278. Diagram of the auditory 

 organ (membranous labyrinth) of a Ver- 

 tebrate, a ampulla, 6 semicircular canal, 

 s sacculus, u utriculus {s + u auditory 

 Tesiole), sg cochlea. — Orig. 



