ADAPTATIONS 409 



young hatch early and are then set free, while the young of the 

 crayfish hatch later and cling to the mother for some time after 

 hatching. 



833. The Amphibia generally deposit their eggs in water, 

 but they avoid the streams which have a strong current, 

 preferring quiet pools, ponds or even stagnant puddles. They 

 also often attach the eggs to objects under water by means of 

 the gelatinous envelope^ which holds the eggs together in masses. 



834. The embryo of the marine Lamellibranchs is set free 

 at an early stage as a free-swimming veliger (page 338). At a 

 corresponding stage the young glochidium of the fresh-water 

 mussels, the Unios and Anodontas, become attached to fishes, 

 where they continue their development for perhaps several 

 months longer before they finally become free. 



835. Lakes and ponds often swarm with many kinds of 

 minute free-swimming organisms, which are comparatively 

 rare in streams. Only the larger forms, with their stronger 

 swimming powers, can make headway against an ordinary 

 current, and are thereby enabled to maintain themselves in 

 the waters of creeks and rivers. 



836. The insect faunas of oceanic islands present a similar 

 phenomenon. These insects are either wingless or, if they 

 have wings, are seldom seen to use them. The explanation 

 offered is very simple. Few insects are able to fly against a 

 strong wind, and strong winds are particularly prevalent on 

 oceanic islands. Under such circumstances if an insect were 

 to rise into the air it would most likely be carried to sea 

 and perish. As a result only those insects which cannot, or at 

 least do not, fly have remained. 



837. Adaptations to Water. Attention may again be called 

 to the important adaptations which have reference to water. 

 These are particularly well exemplified by comparing Hydro- 

 phytes and Xerophytes, or by comparing aquatic and terrestrial 

 animals, especially with regard to the integument. 



