THE GILLS. 1^3 



single row along the fourth arch (cf. Figs. 75 and 83). From 

 their first appearance the internal gills are very vascular, re- 

 ceiving branches from the afferent and efferent branchial vessels, 

 which are connected by capillaries in the gill-tufts themselves. 



The relations remain much the same up to the time of the 

 metamorphosis, the gills forming a series of vascular tufts 

 arranged in double rows along the ventral surfaces of the gill 

 arches (Fig. 75, Gi), and hanging down into the opercular cavity, 

 which they in great part fill. The dorsal or pharyngeal borders 

 of the gill arches develop a complicated system of tooth-like 

 processes, which form a filtering or straining apparatus, pre- 

 venting the passage of food from the pharynx through the gill- 

 clefts. This is still further obviated by a pair of velar plates, 

 anterior and posterior, on each side of the floor of the pharynx, 

 which cover over the gill-arches, and separate them from the 

 pharyngeal cavity ; a rather narrow slit is left between the edges 

 of the two plates of each pair, for the passage of water from the 

 mouth to the gill-clefts, for the purpose of respiration. 



The disappearance of the gills. Towards the end of the tad- 

 pole period of existence, large numbers of lymph follicles form on 

 the inner surface of the opercular membrane ; and at the same 

 time a great proliferation of epithelial cells takes place from 

 the epithelium of the opercular membrane, and from the gills 

 themselves. On the gills the cells become cubical, and then by 

 rapid division form layers several cells thick. In this way, by 

 thickening of its walls, the opercular cavity becomes greatly re- 

 duced in size, and ultimately completely blocked up. The gill- 

 clefts become closed, by fusion of their walls with one another ; 

 and the gills themselves, with the branchial cartilages, and the 

 entire gill apparatus, degenerate and are rapidly absorbed. 



Portions of the ventral ends of the gills persist, even in the 

 adult, as a pair of soft, lymphoid bodies, reddish in colour, which 

 lie at the sides of the larynx, just behind the thyroid bodies, and 

 a little further apart than these. They are sometimes spoken 

 of as tonsils. 



Remnants of the dorsal ends of the gills also persist for a time 

 as a pair of compact lymphoid masses, lying immediately beneath 

 the skin, and just behind the ears ; they usually disappear in the 

 course of the second year. 



