. The biology of the amphibia. Amphibians. 330 THE BIOLOGY OF THE AMPHIBIA lens muscle of fishes. In urodeles there is only a single ventral muscle lying in a papilla. This functions the same as the ventral protractor of frogs and is considered by Tretjakoff (1906) to be homologous with it. The urodele protractor is believed by Plate (1924) to be homologous with the fish retractor, and hence the Amphibia may owe this system to their piscine ancestors. In all Amphibia there is a second muscular system which is not found in fish. This is the tensor chorioideae, a series of meridionally arranged
. The biology of the amphibia. Amphibians. 330 THE BIOLOGY OF THE AMPHIBIA lens muscle of fishes. In urodeles there is only a single ventral muscle lying in a papilla. This functions the same as the ventral protractor of frogs and is considered by Tretjakoff (1906) to be homologous with it. The urodele protractor is believed by Plate (1924) to be homologous with the fish retractor, and hence the Amphibia may owe this system to their piscine ancestors. In all Amphibia there is a second muscular system which is not found in fish. This is the tensor chorioideae, a series of meridionally arranged fibers in the periphery of the ciliary region. Streuli (1925) believes this functions antag- onistically to the protractor, but Beer (1899) and Plate (1924) present evidence to show that it functions in mov- ing the lens in the same direc- tion. The tensor chorioideae becomes the ciliary muscle of the eye in amniotes (Plate, 1924). The extent of accommoda- tion varies with the species but shows some correlation with the species habits. It is apparently greatest in the ter- restrial Bufo and least in such aquatic forms as Bombina. Newts have been reported to be nearsighted on land but not in the water. Retina.—The eye is fundamentally unlike other sense organs in that the retina is not directly evolved from the external ecto- derm but from part of the central nervous system. It arose in phylogeny from an aggregation of direction eyes (Parker, 1908) which were inverted on the development of a tubelike central nervous system. Thus the rods and cones, the only photosensitive cells in the eye, are directed away from the lens and light passes through several layers of nerve fibers and their nuclei as well as much supporting tissue before reaching the sensitive Fig. 119.—Vertical meridian section of a frog's eye, showing muscles of accommodation. C, cornea; iris; L., lens; , M. protractor lentis; , M. tensor chorioideae; , pars ciliaris retinae; R., retin
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