RESPIRATORY ORGANS. 



of the first, are lost. Their presence in this group can only 



be explained as inheritances from branchiate ancestors. The 



first gill pouch in the anura and 



the higher groups form the Eusta- 



chian tube (see ear). 



In elasmobranchs and some ga- 

 noids the anterior visceral cleft is 



smaller than the others, and opens 



on the top of the head. This spir- 

 acle bears well-developed gills in 



the lowest sharks (notidanidae), 



but in others it may have but a 



vascular network in its walls. In 



ganoids and embryonic teleosfs it 



has a gill-like structure ; but it is 



here termed a pseudobranch, since 



it receives arterial blood from the 



opercular gill. The opercular gill 



is a secondary and ectodermal 



structure developed on the inner 



or posterior face of the operculum 



(see below). 



In the typical elasmobranchs the interbranchial septa extend 



to the outside of the body, and the gill clefts open directly to 



the exterior, either on the sides of the neck (selachii, Fig. 26) 



or on the ventral surface (raiae). In the cyclostomes, Myxine 



excepted, there is also a separate opening for each gill cleft. 



In the holocephali a fold of skin on either side grows back over 



the gill clefts, thus en- 

 closing a space into 

 which these empty, and 

 which in turn connects 

 with the exterior by a 

 slit-like gill opening be- 



FiG. 24. Section through the 

 head of a pig embryo 6.5 mm. long, 

 showing the gill slits (1,2,3,) closed 

 by a thin wall. At the left a small 

 portion enlarged. , Eustachian 

 cleft ; //, hyphophysis ; /)/, man- 

 dibular cleft ; P, pharynx. 



FlG. 25. Tadpole of frog, showing 

 the external gill opening. 



hind. In the ganoids 

 and teleosts the same relations occur ; but in these the fold, 

 known as the operculum, has a cartilaginous or bony internal 

 skeleton. In the amphibia the opercular fold is also found, but 



