The mesenchymal layers of the arch of the hyo- quently, the wvolut ionary tronsf ormat i on of the 



branchial skeleton in the larvae of alT~The in- hyobranchial apparatus in the larvae of Urodela 



vestigated forms initially rests in a transverse which are connected with the loss of the breathing 



position. Then, to the degree that there is re- function took place by means of a negative anabol- 



location of the stomadeum from the ventral side ism and the transformation which conditioned the 



of the embryo to the end of the snout (in active- perfection of the mechanism for food capture, by 



ly feeding larvae the mouth is terminal), the means of the positive mode of phyloembryogenes is. 

 hyoid and the gill arches change their position. 

 As a result they are situated obliquely with re- 

 lationship to the longitudinal axis of the body 



(the dorsal end of each arch is located signifi- CHAPTER II 



cantly more caudal than the lower). This creates 



the possibility of turning of the hyoid arch in THE HYOBRANCHIAL APPARATUS OF THE STEGOCEPHAL IDS 

 the sagittal plane and at the same time the whole 



mechanism of action of the hyobranchial appara- The study of the structure of the hyobran- 



tus. chial skeleton in the stegocephal ids was re- 

 stricted to representatives of Labyr i nthodont ia 



Parallel with the change in position of the (groups which are considered to be the ancestors 



arches, the hyobranchial skeleton differentiates f Anura). In the literature there is infor- 



and the muscles which control it develop further. mation on the structure of the hyobranchial 



The copula forms in the medial part of the floor skeleton of representatives of one family of 



of the oro-pharyngeal cavity. In the majority of Lepospondyli ( Lysorophus coctinus ) and four fami- 



the forms investigated it arose immediately in \\ es f Labry i nthodont ia ( Dvi nosaurus . Platyceps f 



the form of a single element. Only in Ranodon Gerrothorax , Tr imerorhach is . Micropholis , Branchio - 



aibiricus at one of the stages of development saurus ). All of these stegocephal i ds are neotenic 



were we able to discover a branchial segmental- forms and possess larval structures of the hyo- 



like formation of the copula, which is en- branchial apparatus, 

 countered only in Acanthodii (Watson, 1937) - 



The branchjale basalia already fuse in the fol- The number, form, and correspondence of ele- 



lowing stage. This shows that the evolution of ments of tne h yo branch ial apparatus (with the 



the basal ia elements of the hyobranchial skele- exception of the well-developed hyobranch iale 



ton involved fusion rather than reduction. This, m in ser j es f representatives from both groups) 



as well as the fusion of the basalbranch, ial ia were the same as in the i arvae f Urodela. The 



and grohyale during the transition from fishes arches of the hyobranchial skeleton were always 



to tetrapods, facilitated a better transmission situated obliquely with respect to the longitudinal 



of the action of the M. sternohyo ideus to the ax j s f the b d y . | n Qyjnosaurus the hyoid was 



hyoid arch. suspended by ligaments from the skeleton (Bystrov, 



1938, 1939). Epibranchials were absent in all of 



In the initially continuous mesenchyme for- the above mentioned stegocephal ids. There were 



mation of the hyoid and gill arches independent three giu sl j tS) the same number as in the lar- 



centers of cartilage are differentiated and vae of ex tant Urodela (the first and the last gill 



these correspond to the later division of the slits of the ancestral tetrapods were absent), 



arches into hypo- and cerato- elements. During | n addition, evidence of external gills was found 



the early stages of development of hynobiid lar- ; n some stegocephal i ds. Dvinosaurus was also 



vae the hypohyalia lie under the edge of the noted to have external gills (Bystrov, 1938) . 



ceratohyalia. This bend disappears as in the The similarity of the structure of the hyobran- 



Rhipidistia (Jarvik, 1954, I963), and the hyoid chial skeleton in both the larvae of stegocepha- 



assumes a definite form for the larvae of lids and the larvae of extant Urodela presumes a 



Urodela. In the larvae of Ambystomidae and similarity in their mechanism of action. The 



Salamandridae the hyoid is formed directly presence of external gills indicates that the hyo- 



without the above mentioned curve. Comm i ssurae branchial apparatus of stegocephal ids as well as 



terminales which unite the posterior ends of the of the i arvae of urodelans serves for the capture 



ceratobranchiali arise as outgrowths from these of food- Thus> we were able to explain that de- 



after the gill arches have been completely spite variations among the several representa- 



formed, that is, their development occurs by tives, the hyobranchial apparatus of the larvae 



means of positive anabolism, extensions of the of Labry i nthodont ia and the larvae of Lepospondyli 



final stages of development of their ceratobran- was cons t ruc ted and functional on the same prin- 



chiali. These elements of the hyobranchial ciple as that of the larvae of the extant Urodela. 

 skeleton are distinctive only for the larvae of 

 Amphibia; the elements are new formations, 

 and hence not homologous to the ep i branch ia of 



fish as suggested by Fox (195*)- B Y means of a CHAPTER III 



positive anabolism the supplementary portions 



of M. subarcularis rect . J_V and of the crest THE EVOLUTION OF THE HYOBRANCHIAL APPARATUS IN THE 



which serve for their strengthening have also LARVAE OF ANURA 

 developed in the larvae of the higher Urodela. 



The reduced parts of the hyobranchial apparatus |. |, Schmalhausen (1964) has suggested that 



(4th and 5th gill slits, hyobranch iale III) the hyobranchial apparatus of tadpoles was formed 



evolved through negative anabolism. Conse- under conditions of rapid flow of mountain streams 



