534 



COMPAEATIVE AXATOMY. 



points witli tlie primitive vesicle, is differentiated from it, and 

 forms a semicircular canal, so tliat the whole labyrinth has a 

 circular form. In the Petromyzontes there ai'e two of these canals, 

 each of which commences with an ampulla-like enlargement, while 

 the rest of the vesicle of the labyrinth forms the "membranous 

 vestibule ; " in this there is a special diverticulum, which is the 

 rudiment of a new differentiation. In the Gnathostomata a third 

 canal is developed, so that henceforward three semicircular canals 

 open into the vestibule. 



When the vesicle of the labyrinth sinks beneath the surface, its 

 stalk-like basal piece remains open on the roof of the skull, in the 

 Selachiij and swells out below the integument into a saccus endo- 

 lymphaticus. This corresponds to the recessus labyrinthi (ductus 

 endolymphaticus), which passes up as far as the roof of the skull in 

 the Teleostei, and may undergo various metamorphoses. One of these 

 metamorphoses has been regarded as leading to the growth of this 

 portion into a tube which covers the brain (Urodela), or extends to 

 the base of it (Anura). In the Ophidii and Saurii it reaches' to the 

 roof of the skull, being filled in the embryo with crystals of lime, and 

 widened out. In Phyllodactylus it extends beyond the skull, and 

 may even pass into the cervical region, being swollen out in parts. 

 The connection between these structures and the primitive stalk of 

 the otocyst is denied, so that the recessus labyrinthi is regarded as 

 an independent structure. Most of its relations, however, require to 

 be more carefully investigated. In Birds it is an open cavity {r J), 

 for a short time only ; so, too, in Mammals, where later on it forms 

 the aqueductus vestibuli. The vestibule and semicircular canals are 

 very large in all Fishes j in the Selachii and Dipnoi they are com- 



rig. 301. Auditory organ of Cyprinus carpio. a. Membranous vestibule, b Ampulla 

 of the posterior and external semicircular canal, c United anterior and posterior canal. 

 (I Posterior, e Anterior. / Oanalis sinus imparls. g Sinus auditorius impar. 

 /; Claustrum. I h I Chain of connecting ossicles, m n Air-bladder o Air-duet. 

 p q r s Spinous processes of the anterior vertebrae. The numbers indicate the different 

 bones of the skull. 1 Basi-occipital. 2 Exoccipital. 3 4 Supra-occipital. 6 Petrosal. 

 7 Parietal. 10 Alisphenoid. 11 Frontal (after E. H. Weber). 



pletely surrounded by the walls of the skull, while in the Teleostei 

 the median portion projects freely into the cranial cavity (Fig. 301). 

 Of the three semicircular canals, two — an anterior (e) and a pos- 

 terior one {d) — are placed in the direction of two planes, which cross 



