AIR-BLADDER. 251 



These external gills are acted on by a complicated system of 

 muscles, and are covered by ciliated epithelium, which serves to 

 keep up a continual current in the surrounding medium. 



In the Axolotl and in the larva) of Salamanders there are four, and in 

 Menobranchus and Proteus only two gill-clefts perforating the pharynx. The 

 former thus show a more primitive condition, while in the latter these 

 structures have become reduced. There is always only a single external 

 opening covered over by an opercular-like fold of skin. 



In the Derotremata the gills disappear entirely, but the gill-aperture 

 between the third and fourth branchial arches persists. 



The external gills present at first in Anura soon disappear, 

 and their place is taken by internal gills. By the growth of the 

 opercular folds, the external respiratory aperture of either side 

 becomes gradually reduced in size, and the two branchial chambers 

 come eventually to open by a single aperture, which is situated 

 either in the median ventral line, or laterally. 



In the larvae of Notodelphys and of Caecilia compressicauda bran- 

 chial vesicles are developed, covered over by a vascular respiratory network ; 

 in the former these are bell-shaped, and in the latter they have an irregular 

 sac-like form. In the embryo of Epicrium glutinosum, a feather-like and 

 highly-vascular gill-tuft arises on either side ; these are of unequal length, and 

 they move continually backwards and forwards in the egg-albumen. In certain 

 Batrachia, the broad and richly vascular swimming-tail lying against the 

 egg-membrane may serve as a larval respiratory organ. 



Thus we arrive at the result that the gills of Vertebrates may 

 be divided into four groups, which show no direct connection with 

 one another. The first kind is seen in Amphioxus, the second 

 in Cyclostomes, the third in the adults of other Fishes, and 

 the fourth in Amphibians. 



II. AIR-BLADDER AND LUNGS. 



1. THE AIR-BLADDEE, 



As has already been mentioned, the lungs and air-bladder are 

 developed in a similar manner, and differ only from one another in 

 the fact that the lungs always arise from the ventral side of the 

 primary oesophagus, while this is an exceptional case as regards 

 the air-bladder (Poly pterus, Erythrina) ; that organ is usually 

 formed on the dorsal side. The exact point of origin of the 

 air-bladder from the oesophagus varies, and its duct (duet us 

 pneumaticus) may either remain open throughout life, as in 

 all Ganoids and some Teleosteans (Physostomi), or it may 

 later become reduced to a solid fibrous cord, or even entirely 

 obliterated, as in other Teleostei (Physoclisti). In the latter 

 case there is no communication between the air-bladder and the 



