XII.] 



THE EXCRETORY ORGANS. 



479 



In the Tenth Lesson has been already described the mode 

 in which the blood is brought to the gills from the heart, and 

 from the gills to the general circulation, as also the changes 

 which, in Batrachians, accompany the disappearance of the 

 gills during the developmental metamorphosis. 



The blood is carried in osseous Fishes (e.g. the Perch) 

 along the convexity of each branchial arch both to and from 

 the gill-filaments, the margins of which are skirted by 

 arterial and venous branches, capillary vessels being inter- 

 posed between them. 



FIG. 405. Two LAMELL.-E (OR LEAFLETS) FROM THE GILLS OF AN OSSEOUS FISH, 

 showing the course of the respiratory circulation. 



s, cut surface of one of the branchial arches. On its upper side is seen a concavity 

 which is produced by the section of the groove which runs along the convex 

 and exterior (here upper) side of each branchial arch, bn, branchial arttry in 

 section, giving oft" the gill-arteries (go) to the adjacent sides of the gill-leaflets, 

 whence the blood is distributed in the leaflets : gv, the gill-veins which run 

 along the outer side of the gill-leaflets, collecting the blood from them by 

 minute veins and pouring it into bv, the branchial vein, which runs down the 

 groove of the branchial arch and has the branchial artery superficial and 

 exterior to it. 



Gills which are at first conspicuous but mostly dfsappear 

 ultimately, are termed external branchice. Such are absent in 

 almost all adult Fishes, though often present in the young con- 

 dition, e.g. in young Sharks. In Batrachians, such as the Tad- 

 pole, they also disappear and give place to internal gills ; but 

 in some forms, e.g. Axolotl and Menobranchus, they persist 

 throughout the whole of life as long plume-like appendages 

 placed on each side of the neck. 



