24 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



in urodeles and caecilians it develops but slightly. In the anu- 

 ran tadpole, on the other hand, the opercular folds of the two 

 sides unite beneath the throat, thus connecting the extra bran- 

 chial chambers of the two sides, and then the folds unite to the 

 sides of the body, usually leaving but a single opening on the 

 left side through which water is discharged from both right 

 and left gills 1 (Fig. 25). 



In the cyclostomes the gill slits are narrow tubes widened 

 in the middle into a saccular shape (whence the name marsipo- 

 branchs, pouched gills, often given the group). In these sacs 



A B 



FIG. 26. Relations of gill clefts, etc., in an elasmobranch, A, and a teleost B. 



are the demibranchs. In the elasmobranchs the septa extend 

 to the external surface, the gills not extending so far. In 

 ganoids and teleosts, on the other hand, the septa are reduced 

 to small rods while the demibranchs are greatly enlarged. 



In the embryonic amphibia external gills occur. These 

 are ectodermal structures developed from the outer surface of 

 the gill septa' 2 even before the gill clefts break through. In 



1 No operculum is developed in the amniotes ; but there is some plausibility in the 

 view which regards the external ear of the mammals as a derivation of the ichthyopsidan 

 operculum. 



2 Relations, blood supply, and nerves go to show that the fleshy processes (so called 

 balancers) of the urodele larva? are the modified external gills of the hyoid arch (see 

 Fig. 199). 



