YOUNG OF REPTILES 



and with abundant yolk. Correlated with the large 

 size of the egg the young when hatched from it is prac- 

 tically adult. Beyond a slight tooth developed upon the 

 snout for the purpose of breaking the eggshell it is like 

 its parent. There is nothing at all comparable to the 

 larval forms of the amphibia. In these latter animals 

 the eggs are not large and have not abundant yolk. 

 Correlated with this is a condition unlike that of the 

 reptilia. The young are born in an earlier condition ; 

 and inasmuch as in that earlier condition of necessary 

 incompleteness the young would be unable to lead an 

 independent existence, they have been provided with 

 certain organs which are of use to them only as young 

 and which disappear when they reach maturity. To 

 the young of an animal which leaves the egg in an im- 

 mature condition, but which has special organs suited to 

 a free life in that condition which later disappear, the 

 term " larva " is applied. Many groups of the animal 

 kingdom go through a larval stage. For instance the 

 butterfly leaves the egg as a caterpillar, which is a larva. 

 The caterpillar is not merely an imperfect butterfly. 

 It has special organs of its own. Its jaws for example 

 are not imperfect jaws which later develop into the 

 trunk and so forth of the butterfly. They are complete 

 as biting jaws and disappear when the adult condition 

 is assumed. So, too, the tadpole of our common frog, 

 being hatched at a stage antecedent to the appearance 

 of the later developed lungs, breathes by means of 

 external processes of the skin, which are the gills and 

 peculiar to the larvae, disappearing when it metamor- 

 phoses into a frog. Below the jaw is a sucker which 

 allows it to moor itself to water plants. This structure 

 is not the immature form of any structure which occurs 

 in the frog. It is an organ only found during the tad- 

 pole stage ; it is in fact a larval organ and its existence 

 is one of the reasons for terming the tadpole a larva. 



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