114 THE STORY OF LIFE IN THE SEAS. 



characters of larval forms among terrestrial and 

 air-breathing animals, but in none of these do we 

 find so great a specialisation in larval characters 

 as in some of the marine forms of life. 



It is said above that the Echinoderms as a 

 general rule have free-swimming larvae, but there 

 are exceptional cases which have an interest for us 

 quite as great as that of the ordinary life-history. 



Many Echinoderms are found living in very 

 great depths of the ocean and it is difficult for 

 us to believe that any of these can have pelagic 

 larvse similar to those of their shallow water 

 relatives. The difference in pressure between 

 that of the bottom of the deep sea and of the sur- 

 face is, by itself, sufficient to convince us that a 

 delicate organism like a Brachiolaria or Pluteus 

 could not make the upward journey unharmed ; 

 but when we add to that the great distance of 

 two or even three miles in a direct line, the dif- 

 ference in temperature and in light, we must 

 realise that the ordinary transformations of the 

 shallow water Echinoderms is an impossibility 

 for the deep-sea varieties. 



As a matter of fact we know very little about 

 the life-history of deep-sea Echinoderms, and this 

 is not a matter for wonder when the reader re- 

 flects upon the great difficulties that have to 

 be overcome in obtaining a few specimens of the 

 adult forms ; but at least one of the Star-fish of 

 the Abyss has been found to bear little pouches 

 or pits in which the young are fostered until 

 they are ready to lead an independent life in the 

 form of the parents. 



It has also been shown that in some of the 



