790 LIFE : OUTLINES OF GENERAL BIOLOGY 



of metamorphosis before it becomes an adult. The caterpillar 

 is beginning its metamorphosis before it enters the chrysalis or 

 pupa stage, in which it transforms fully to butterfly or moth. The 

 tadpole is metamorphosed — and also during a period of fasting — 

 into the likeness of a frog or a toad. The free-swimming larvae of 

 starfishes and sea-urchins have to undergo metamorphosis before 

 they show any likeness to their parents. 



The antithesis to the occurrence of larvse in a life-history is direct 

 development, such as is familiar in ordinary reptiles, birds, and 

 mammals, or in earthworms, leeches, land-snails, and cuttlefishes. 

 We say "ordinary" so as to leave it an open question whether a 

 new-born marsupial, for instance, does not almost deserve the 



Fig. 135. 



Pelagic Phyllosome or Glass-Crab Larva of the Rock-Lobster, Palinurus. 



After Roiile. 



designation larva. It used to be a favourite idea that direct develop- 

 ment represented a secondary simplification of a long-drawn-out 

 life-history with larval stages. Thus the life-history of a cockroach, 

 where a miniature adult breaks out of the egg-shell, was regarded 

 as less primitive than the typical life-history of all the higher insects, 

 where a larva emerges from the egg, and in the course of a quiescent 

 pupal phase is changed into the winged imago, structurally very 

 different. There are probably some cases where this reduction of 

 larval stages has secondarily occurred, so that a telescoped life- 

 history has resulted; but on the whole the direct type of develop- 

 ment is primary, and the interpolation of larval stages is secondary. 

 The commonly held view nowadays is that larvae represent 

 secondarily specialised growth-developments arising at some 

 punctuated phase in a direct life- history. The caterpillar is not an 



