. Elements of Comparative Anatomy. 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 i
. Elements of Comparative Anatomy. 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 posteri
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