34 THE BIOLOGY OF THE SEASONS 



branchial arches. These are not comparable to the ex- 

 ternal gills of a young shark or skate, which are really 

 elongated internal gills projecting through the gill-clefts. 

 They are comparable to the true external gills seen in the 

 young of some very archaic fishes, still living to-day, the 

 Polypterus of the Gambia and some other African rivers, and 

 two of the mud-fishes, Protoptems from Africa, and Lepido- 

 siren from the Amazons. One or two days after hatching 

 (in our common Rana temporaria) another important 

 structure appears on the larva the mouth is formed in the 

 centre of a groove in front of the adhesive organs, and 

 hundreds of small horny teeth are developed. 



When the food-canal becomes open, four pairs of gill- 

 clefts break out from the pharynx, and a gill-cover overlaps 

 the first set of gills. These dwindle and are absorbed, their 

 place being taken by a second set of gills supported by the 

 hinder margin of the lower halves of four gill- arches. As 

 these are enclosed in a gill-chamber and as they form a 

 second set, it is natural to compare them to typical fish-gills. 

 But they are really in the strict sense external, and they are 

 certainly skin-covered. Each gill-chamber has at first its 

 opening, but that of the right side joins with that of the left. 



About a month after hatching the larval frog is in many 

 ways fish-like : for instance, it has a two-chambered heart 

 which drives impure blood to the gills, which are enclosed by 

 a gill-cover. It swims by its laterally compressed tail, 

 which shows a well developed unpaired fin, without fin-rays, 

 however, which support the unpaired fins of fishes. In a 

 very general way it may be said that the developing frog 

 visibly climbs up its own genealogical tree. It is this 

 general idea, indeed, of recapitulation that makes the study 

 of the frog's life-history perennially interesting. It re-enacts 

 the epoch-making colonisation of the dry land, and in many 

 of its internal changes, e.g. in the making of the three- 



