BREATHING ORGANS. 



279 



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

 covered by a membranous operculum (fold of skin growing back 

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



OF 



BF 



OS 



3 R.I 



BR.2 



KA 



KA 



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

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

 second, BRB third branchial arch ; C body-cavity ; EF efferent vessel of first bran- 

 chial arch; HM hyomandibular 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 ; TI intestinal region of enteron ; Y 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 



