BREATHING ORGANS. 



279 



the Urodela ; but in the Anura in which the gill-openings become 

 covered by a membranous operculum (fold of skin gro\^dng back 

 from the hyoid arch), the external gills atrophy and are replaced 



3R.1 



BR.2 



Fig. 154. — Horizontal section of the head ol a tadpole at the time of hatching x 40 (after 

 Marshall). AF afferent vessel of first branchial arch ; BF fore brain ; BRl first, BR2. 

 second, BR3 third branchial arch ; C body-cavity ; EF eflterent vessel of first bran- 

 chial arch ; HM hyomandibuJar pouch ; HY hyoid arch ; IN infundibulum ; KA 

 archinephric duct of right side, KA' of left side ; KP pronephros ; KS third opening of 

 right pronephros, KS' ditto of left pronephros ; OF olfactory pit ; OS optic stalk ; TP 

 pharynx ; Tl intestinal region of enteron ; F yolk cells. 



by a series of very similar but smaller structures developed 

 along the more ventral portions of all four branchial arches. 

 These are the internal gills, so called from their position beneath 

 the operculum and not from any essential difference between 



