the musculature of the hyobranchial apparatus of 

 tadpoles occurred primarily through positive ana- 

 bolism which could be clearly traced in a whole 

 series of instances. The transformation of the 

 M. i ntermand i bular i s poster i or and M. s ternohyo id - 

 eus occupies a particular place. These muscles 

 play a leading role in the mechanism of action 

 for the hyobranchial apparatus in the larvae of 

 Urodela, but do not function in tadpoles; for 

 this reason they are always incompletely developed 

 although they have not disappeared completely in 

 any of the investigated forms. The preservation 

 of the primordia for M. i ntermand i bular i s post - 

 er ior and M. sternohyo ideus is explained by the 

 fact that they induce movements of the hyo- 

 branchial apparatus in adult Anura (A. S. 

 Severtzov, 1961). Consequently one observes 

 not a reduction but a retardation in anuran 

 larvae, a temporary restraint in the develop- 

 ment of these muscles which is elicited by 

 their exclusion from the mechanism of action 

 of the larval hyobranchial apparatus and by 

 the necessity of preserving them for future 

 function. Speaking more generally, retarda- 

 tion may begin when any component is excluded 

 from the functioning of the provisional or 

 temporary system even though the component is 

 preserved for functioning at subsequent stages 

 of ontogenesis. The offered conception to a 

 certain degree explains the adaptive signifi- 

 cance of regulated retardation in ontogenesis - 

 a very little studied class of heterondria. 

 Thus in the larvae of Anura a mechanism of 

 action of the hyobranchial apparatus is developed. 

 The evolution of the latter proceeded primarily 

 by means of positive modes of phyloembryogene- 



HYOBRANCHIAL APPARATUS OF RH I P 1 1ST I A 



In the investigation of mechanism of action 

 of the hyobranchial apparatus in the larvae of 

 Amphibia, the elaborate descriptions and recon- 

 structions of the structure of the hyobranchial 

 apparatus of Rhipidistia (jarvik, 1954, I963, 

 1967) give us the possibility to reconstruct the 

 mechanism of action for this systea in the 

 crossopteryg ian fishes and also to weigh the 

 arguments in favor of the theory of polyphyle- 

 tic origins of the tetrapods advanced by Jarvik. 

 The structure and location of the pulmonary discs 

 in Glyptolepis and Holoptych ius ( Porolep i formes) 

 and in Eusthenopteron ( Osteolep if ormes) is such 

 that they condition the mobility of the floor of 

 the oro-pharyngeal cavity and hence the rising 

 and falling movements that assure the oscilla- 

 tion of the hyobranchial apparatus. The structure 

 of the hyobranchial skeleton of Eusthenopteron 

 and in particular of Glyptolep is is similar to 

 the structure of this system in the larvae both 

 of the stegocephal ids and of the extant Urodela 

 (the most important shared characteristic is 

 the oblique or slanting location of the hyoid 

 and gill arches which shows that the oscillations 



of the hyobranchial apparatus must have taken place 

 in the sagittal plane during the action of the M. 

 sternohyo i deus ). Consequently the mechanism of 

 action of the hyobranchial apparatus in Rhipidis- 

 tia, which served primarily for breathing, was 

 similar to such mechanisms in the larvae of the 

 stegocephal ids and the larvae of the extant Urodela, 

 but not in the larvae of Anura. One of the argu- 

 ments used to substantiate the theory of the 

 polyphyletic origin of the tetrapods advanced by 

 Jarvik (1963, 19^7) ' s based on his homology of 

 a series of elements of the hyobranchial skeleton 

 of Eusthenopteron and the larvae of Anura. In 

 his opinion the sublingual pivot is homologous 

 to copula I, the bas i branch iale I to copula I I 

 and the bas i branch iale II to the median fissure 

 of the hyobranchial disc. The cited homologous 

 features allowed Jarvik to come to the conclusion 

 that the hyobranchial apparatus of the larvae of 

 Anura developed on the basis of the structure of 

 this system in Eusthenopteron . We succeeded in 

 showing that of the homologous features advanced 

 by Jarvik only the comparison of bas i branch iale 



I in Eusthenopteron and of the basal elements of 

 the hyobranchial skeleton of the tadpoles copula 



II was justified. Copula I is a new formation 

 which is characteristic only of the larvae of 

 Anura (see chapter 3) a "d the medial fissure of 

 the hyobranchial disc arose during the process of 

 the proliferation and fusion of the hypohranch ia - 

 1 ia and does not have anything in common with 



the basal elements. Nor is it possible, as 

 presumed by Jarvik, to reconstruct the muscula- 

 ture of the hyobranchial apparatus of Eusthenopteron 

 using the correspondences to analogous muscles 

 in the larvae of Anura; all that is further em- 

 phasized by this is the similarity of the forms 

 compared. The muscles of the hyobranchial appara- 

 tus of the tadpoles are highly changed as a result 

 of the origin of a new mechanism of action. The 

 presence of a sublingual pivot (which is not 

 known in any of the extinct or contemporary forms) 

 and a series of other characteristics in the con- 

 struction of the hyobranchial skeleton attest to 

 the specialization of Eusthenopteron . Thus the 

 structure of its hyobranchial apparatus cannot 

 be considered as a prototype for the structure of 

 this system in the larvae of Anura and it is not 

 possible to confirm or establish the theory of 

 the polyphyletic origins of the tetrapods by 

 means of such comparison. 



CONCLUSIONS 



In 



-y we 



factors which determ 

 hyobranchial apparatu 

 in the larvae of Amph 

 hyobranchial apparatu 

 cephalids and the lar 

 is characterized by t 

 i n brea th i ng and i ts 

 for prey capture. Th 

 duction in a series o 

 branchial skeleton an 

 several gill openings 



st characterize the basic 

 ne the transformation of the 

 s and its evolutionary trends 

 ibia. The evolution of the 

 s in the larvae of stego- 

 vae of the extant Urodela 

 he loss of its function 

 perfection as a mechanism 

 e loss has permitted the re- 

 f elements of the hyo- 

 d the disappearance of 

 , as well as several 



