METAMOEPHOSIS OF THE TADPOLE. 701 



dilate, and a freer supply of blood is poured into the pulmonary system ; 

 until at last, when the lungs are fully formed, and the branchial arteries 

 (a a a) and veins (///) quite obliterated, all the blood necessarily passes 

 immediately through the anastomotic trunks (e e e), which of course then 

 represent the vessels (o o o) of the Menopoma (fig. 343) ; and the mode 

 of respiration is thus completely converted from that of a Fish into that 

 of a true Reptile. 



(1966.) But during the progress of these changes in the disposition 

 of the vascular system, others not less wonderful take place in the form 

 and uses of the entire hyoid apparatus, and in those muscles of the throat 

 which are connected with the function of respiration. 



(1967.) The hyoid apparatus of the tadpole is, in fact, a very com- 

 plicated structure*, and, like that of the fish, supports the branchise, 

 and facilitates the entrance and expulsion of the water ; moreover, by 

 opening or closing at pleasure the communication which exists through 

 the branchial apertures between the mouth and the exterior of the body, 

 it thus allows air to be taken into the lungs at pleasure. 



(1968.) The os liy aides of the tadpole, at an early period of its de- 

 velopment, supports four branchial arches (fig. 346, A, 1, 2, 3, 4), which 

 bound three branchial fissures, through which, as in a fish, the water 

 escapes from the mouth. The branchial arches 2 and 3 are studded on 

 each side with cartilaginous points, and the arches 1 and 4 have similar 

 points on one side only ; so that when the arches are approximated, as 

 they can be by an elaborate temporary set of muscles provided for the 

 purpose, the cartilaginous teeth lock into each other so accurately that 

 the branchial fissures are completely and firmly closed a provision 

 which is evidently indispensable in order to allow the tadpole to fill its 

 lungs with air. 



(1969.) The above is the condition of the branchial portion of the 

 hyoid apparatus before the metamorphosis of the tadpole has made 

 much progress ; and from this time a series of changes begins of a most 

 curious and interesting description. 



(1970.) When the metamorphosis has commenced, the os hyoides 

 and branchial arches assume the appearance represented at fig. 346, B. 

 The pieces 8 and 9 are no longer both cartilaginous, the latter having 

 become entirely ossified. The branchial arch 1 is likewise converted 

 into bone ; and its upper surface, being considerably enlarged, is now 

 connected with both the pieces marked 10 and ll. The three carti- 

 laginous pieces 5, 6, 7, in fig. 346, A, are consolidated into one, while 

 the branchial arches 2, 3, 4 become much reduced in size, the branchias 

 approach each other, and the cartilaginous points with which they are 

 provided adhere together, so that from hour to hour, so to speak, the 



* Recherches anatomiques et physiologiqu.es sur les organes transitoires et la me*ta- 

 morphose des Batraciens, par J. Gr. Martin St.-Ange (Annales des Sciences Naturelles, 

 xxir.). 



