CLASSIFICATION 41 



Hydra a two-layered bag of cells, of which the 

 inner layer, lining the cavity, performs the work 

 of digestion. The lowest of the 

 Vertebrata, the Lancelet (see p. ^ EC 



140), has a larva of this kind. 

 The same reasoning which sug- 

 gests the existence of an ances- 

 tral Gastraea-animal, suggests 

 that of an ancestral Planula- 

 animal ; for the two-layered ani- 

 mals, on their part, present us 

 with a monoblastic larva of the 

 form already described (p. 34), FlG . 6. Diagram- 



Called a Planula. Hence it is matic representation 

 that zoologists look With SUCh Vr^two^ered 



eagerness for forms, of which it larval form, highly 

 can be said that they consist of magnified; optima 

 one layer of cells only. The 5?% gg 

 name Planula signifies "wander- or skin layer; En, 

 ing animal," because the Planula JSSJS^JS 

 larva swims about by means of leading into'the en- 

 Cilia, teric cavity. The 



Mention has been made SS^'SaT nudei 

 above of larval forms. It is 

 perhaps advisable to explain clearly what is meant 

 by this term. It is a matter of every-day knowl- 

 edge that in some animals the young form pre- 

 sents an appearance and structure very differ- 

 ent from that of the grown-up form, and adapted 

 for a different mode of life; the commonest in- 

 stances are the caterpillar of the butterfly and 

 the tadpole of the frog. We are apt to think of 

 these creatures as somewhat exceptional in this 

 respect. But the zoologist, in viewing the whole 

 range of the animal kingdom, finds avast number 

 of animals with larvae, differing much from the 



