110 THE STORY OF LIFE IN THE SEAS. 



a portion of their lives in a free swimming 

 condition, and then a change occurs during 

 which they sink to the bottom and gradually 

 assume the adult characters. 



Nearly everybody is acquainted with the 

 general appearance of the Crab and Star-fish, 

 but few would guess that the young stages of 

 these animals are to be found among the minute 

 transparent floating Fauna of the surface waters 

 of the sea. 



The habits of the young and of the old, of 

 these animals are widely different; the former 

 must constantly support themselves in the 

 water, they must feed upon and have means 

 for catching and devouring minute floating 

 organisms and must in other ways be adapted 

 for life with the Plankton ; the latter being 

 unable to swim are capital crawlers and walkers 

 over the rocks and sand of the bottom, have 

 heavy bodies which sink rapidly in the water 

 and, in other ways, are adapted for life with 

 the shallow-water Benthos. 



The conditions of life at the surface and at 

 the bottom being, as I have previously pointed 

 out, so different and the adaptations of structure to 

 suit each set of conditions so great, we have, as 

 a result, a long series of animals in which the 

 young larval stages of life are absolutely 

 unlike the adult and mature stages. 



No better examples to illustrate these changes 

 could be given than those chosen from the group 

 of the Echinoderms. Take, for instance, the 

 common Star-fish with its thick heavy skin 

 studded with plates of carbonate of lime, and 



