GILLS OF BATRACHIA. 515 



( perennibranchiate ' Batrachians, arteries are developed from the 

 last pair of branchial vascular arches, as at p, fig. 344, to convey 

 blood to the luno-s. 



o 



In Menopoma and Amphiuma the gill-opening persists on each 

 side ; but of the original four pairs of vascular arches only the 

 second and third accompany remains of branchial arches, circum- 

 scribe the gullet, and reunite behind to form the beginning of 

 the aortic trunk. The extent of the ossification of the hyo- 

 branchial framework in some measure corresponds with the degree 

 in which the branchial organs of respiration are retained. In 

 Proteus the ceratohyals, urohyal, two pairs of basibraiichials, and 

 three pairs of ceratobranchials become bony. In the Siren the 

 ceratohyals, urohyal, one pair of basibranchials, and two pairs of 

 ceratobranchials are bony. The Menopome and Cryptobranchus 

 agree with the Newts and Salamanders (fig. 333), in having the 

 basihyal, w, the ceratohyals, x, x, and two pairs of cerato- 

 branchials, y and z ; but the latter pair is proportionally longer 

 and shows two elements of the arch, on each side, ossified. In 

 the Anourous Batrachians the branchial arches are reduced to the 

 basal portions of a single pair of ceratobranchials (p. 91, fig. 74), 

 which persist, in most higher Vertebrates, as the ' posterior 

 cornua ' of the f hyoid bone.' 



The parts of the branchial framework in immediate relation 

 with the support of the deciduous gills never pass beyond the 

 cartilaginous stage ; and a histological test is thus afforded of the 

 temporary or permanent character of the branchia? in a Batrachiari 

 presenting them. The deciduous gills offer many modifications 

 in the larvae of the caducibranchiate species. In a tropical South 

 American Frog ( Opisthodelphy s ovifera), e. g., the external gills 

 are formed before the larva is excluded, and expand into a broad 

 membranous disc at their extremity. 



But whatever the form or structure of the external gills, they 

 are fitted only to their office as such : that being discharged, they 

 turn to no other use, but lose their ciliated and vascular structure, 

 and disappear. The Tadpole, meanwhile, being subject to a 

 series of changes in every system of organs concerned in the daily 

 needs of the coming aerial and terrestrial existence, still passes 

 more or less time in water, and supplements the early attempts 

 at pulmonary respiration by pullulating loops and looplets of capil- 

 laries, fig. 345, , from the branchial vessel, b, e, supported by the 

 cartilaginous arch, c, and coated by delicate non-ciliated epithe- 

 lium : the terminal processes of these ( internal gills ' support a 

 single capillary loop, d. They resemble the commencing gills of 



LL 2 



