INTRODUCTION 



The present work is a part of the investiga- system in amphibia. Second, such investiga- 

 tion being conducted in the laboratory of Evolu- lion can shed light not only on the origin 

 tionary Morphology in the Zoological Academy of of the hyobranchial apparatus in the larvae 

 Sciences, in the Academy of Science of the USSR. f the extant Urodela and Anura but also on 

 It deals with the problem of the origin of terres- those processes which led to the structures 

 tial vertebrates which "pertains to the most seen in Recent forms, 

 complex and interesting divisions of phylogeny 



and relates to many general questions of evolu- The problem in the present dissertation 



tion" ( Schmalhausen 1964) . In these investiga- ■ i s the explanation of the following questions: 



tions by methods of comparative embryology we 1. What function or functions does the hyo- 



studied a series of systems of organs with the branchial apparatus fill in the larvae of 



goal of explaining the transformation which took Amphibia? 2. How is this function manifested, 



place in the course of the evolution of verte- in other words, what is the mechanism of 



brates from the ancestral Tetrapoda to contem- action of the hyobranchial apparatus? 3. 



porary Amphibia. The morphob iolog ic and the How did the evolution of the hyobranchial 



functional analysis (with the addition of data apparatus take place in connection with its 



from paleontology) permits us to understand the functions? 4. What was the origin of the 



adaptive significance of the transformations in mechanism of action for this system? 

 organization, and thus to point out and explain 



the basic directions of evolution in the lower The resolution of the principal questions 



Tetrapoda. The latter point is particularly was facilitated by the fact that in the 



significant since there are now two mutually literature the functions and in part the 



exclusive theories on the origins of the mechanism of action for the hyobranchial 



Tetrapoda. According to the theory of mono- apparatus of fish, primarily Teleostei, have 



phyletic origin of the terrestrial vertebrates, been described. The construction of the hyo- 



the Tetrapoda arose as a single branch from branchial apparatus is known for both fishes 



the rhipidistian crossopteryg ian fishes. All and the larvae of amphibians to the degree 



of the variations in extinct and extant forms that homology of the majority of its ele- 



is based upon their adaptive radiation into the ments ; s n0 i onger in doubt. Also, the 



terrestrial and aerial sphere of life. On the basic features of the phylogenetic connec- 



other hand, the majority of advocates of the tions of the lower tetrapods have also been 



theory of polyphyletic origin of the Tetrapoda, explained. In addition, the mechanism of 



which was worked out primarily by the Swedish the expulsion phase of the buccal pump has 



paleontological school, consider that those been reconstructed by M. M. Voskoboen ikov, 



amphibians which have tails and legs arose from (1932) and I. I. Schmalhausen, (196*). The 



one of the branches of Rhipidistia, i.e., the mechanism for the suction phase in the res- 



Porolepiformes, and the tailless amphibian, piration of crossopteryg ian fish is facili- 



Gymnophiona, and all the amniotes arose from tated by the movements of the hyobranchial 



another branch, the Osteolep if ormes. apparatus, the structure of which was des- 

 cribed by Jarvik ( 1954, I963, I967). 

 Our present work deals with the evolu- 

 tion of the hyobranchial apparatus in the 

 larvae of Amphibia and investigates the trans- 

 formation which took place in the course of MATERIALS AND METHODS 

 development of the lower Tetrapods. By the term 



hyobranchial apparatus, we mean that complex of Larvae of four families of Urodela and 



morphologic structures which occupies the area s j x families of Anura were investigated, 



from the hyoid to the last gill arch inclusive, Following is a list of species ( Ranodon 



and all the muscles connected therewith. Onto- sibiricus . Hynobius keyserlinqi i . Ambystoma 



genesis of this morphof unct ional system is con- sp#} Tri tutus cristatus , I. bulqaris . 



eluded at the stage at which the larval hyo- Pleurodeles waltl i i . Salamandra maculosa , 



branchial apparatus is functioning and its Necturus sp., Proteus anquineus . Xenopus 



morphogenesis ceases before metamorphosis. The laevj_s, Bombina bombina . B. orientalis . 



evolution of the hyobranchial apparatus in the Pelobates fuscus, Rana temporaria , R. 



larvae of Amphibia as a single unit in connec- esculenta , Bufo bufo . B. viridis , Hyla 



tion with its function, which has not been in- arborea, H. japonica ). The structure and 



vestigated by anyone prior to this time, should development of the hyobranchial apparatus 



be interesting for two reasons: first, from the was studied in slide preparations and in 



time the vertebrates came up to dry land the a serie s of sections from which were made 



basic direction ,n the evolution of adult m icrophotographs, sketches, graphic and 



Amphibia has consisted of their adaptation to plastic reconstructions. Four hundred and 



the terrestrial and aerial spheres. The larvae twenty-three larvae at various stages of 



of most Amph.bia have continued to live in water. development were examined with morphologic 



he study of the hyobranchial apparatus in the methods. Analysis of functions and of the 



larvae hence gives us material which is more mechanism of action of hyobranchial appara- 



favorable for explaining the origin of this tus was carried out on the basis of obser- 



