TBE FROG 327 



and even on the trees. Their gills have withered away, and the 

 gill cavities now function as lungs. The hard shell, which 

 served as a coat of mail to ward off attacks of enemies in the 

 sea, serves the land crabs to prevent the loss of water. Again, 

 the ancestors of the wood-lice are common in streams, and even 

 in this situation are already provided over the whole body 

 with a thick skin called the cuticula. The possession of this 

 cuticula has permitted the development of land life in this 

 group. The gills, which are thin-walled plates in the aquatic 

 forms, become hollowed out to form lungs. In the case of 

 snails the thick skin, which is useful in secreting the shell of the 

 aquatic forms, has permitted some of its possessors to live on 

 land. Certain representatives of the group, namely, the slugs, 

 have lost the shell without suffering any disadvantage, because 

 they have a very thick skin. Even with the thick skin, how- 

 ever, slugs do not risk a very dry situation, and if forced to 

 remain long in dry air they varnish themselves over with a 

 water-tight layer of mucus. In the slugs, as we have already 

 seen, the gill-chamber has been transformed into a lung-cham- 

 ber by the degeneration of the gills and the diminution of the 

 gill-opening so that the influx of air may be reduced or in- 

 creased as it is dry or moist. 



Way have any aquatic organisms given rise to terrestrial 

 descendants, in spite of the fact that there is room in the 

 sea for all, and food is almost inexhaustible ? The probable 

 answer to this question is that some species whose structure 

 permitted land life found it advantageous to leave the water 

 from time to time to escape from enemies. In the new field 

 they obtained an abundance of oxygen and food, so they pro- 

 longed their stay there until, finally, they became better 

 adapted to the land than to the sea. 



