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Olfactory organ in amphibian and its phylogenetic significance

I. M. Medvedeva

In present monograph is shown on the example of the olfactory organ and structures associated with it the significance of the detailed embryological investigations of separate organ systems for elucidation of the course of phylogenesis and origin of the Tetrapoda. Comparative embryological and comparative experimental methods were applied to the study of 7 species of the Urodela related to families: Hynobiidae, Ambystomidae, Proteidae and S'alamandridae as well as of 7 species of Anura related to families: Pipidae, Discoglossidae, Pelobatidae, Ranidae and Bufonidae. Special attention is paid both to structures newly developed in the process of vertebrates transition from aquatic life to terrestrial existence and to those which radically changed their functions in the course of transition. The choanae, Jacobson's organ and the nasolacrimal duct are namely those particular structures within the system of the olfactory organ.

It is shown that evolutionary rearrangement of the mentioned structures are determined by a single process in which the leading role had been attached to the migration of the nostrils from the head's dorsal side to the edge of the oral cavity. This phenomen caused a phylogenetic rotation of the olfactory sac about its long axis, a connection of the infraorbital seismosensory canal with the sac and a shift of the hind nostrils within the oral cavity (fig. 78). These events made the olfactory organ of Crossopterygii able to function in an aerial environment.

The course of development of the olfactory organ shows that notwithstanding the diversity of its separate structure in Urodela, Anura and Apoda any of the sections within the organ's system ascends to a single prototype. The complex process of the formation of choanae, Jacobson's organ, the naso-lacrimal duct and of the dermal bones lacrymale and septomaxillare connected with the latter are respectively reduced to a single sourse for all amphibia and, hence, testify in favour of their origin from common forerunners.

The emergence of vertebrates onto land had been possible only due to fact that during the period of the aquatic Jife their ancestral forms had already evolved organisational prerequisites in a number of vitally important systems of organs which simultaneously were suitable for the exercising other functions in an other, aerial environment. In the system of the olfactory organ the such are hind nostrils fit to function as choanae, the additional olfactory sac suitable to function as an aquatic olfactory organ in an aerial environment, the segment of the infraorbital seismosensory canal fit to function as a naso-lacrimal duct. Hence, the presence of preadaptations to terrestrial life is explained in a natural scientific way, namely, by multifunctional potency of adaptations. For the implementation of possibility of such radical change of environment, which experienced by vertebrates during their emergence onto land, it was necessary a simultaneous development of such multifunctional adaptations in all the vitally important systems of organs. The probability of this simultaneous arising of adaptations is so small that it fails to allow a repeated conquest of land by Tetrapoda and, hence, along with numerous factual proofs supports the theory of their monophyletic origin.

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