100 THE STORY OF LIFE IN THE SEAS. 



appearance of the Whelks and Periwinkles, and 

 other Gastropods of our shores might be well 

 astonished when he saw, for the first time, many 

 of the Gastropods of the high seas. The shell is 

 either absent altogether or consists of a thin 

 little papery cap far too small to afford protection % 

 to the body, The head and foot, and, indeed, 

 the greater part of the body, are transparent, 

 soft 'and gelatinous like a Jelly-fish, in fact the 

 whole appearance is so different that it is not until 

 the internal anatomy is carefullj studied that 

 their true position in the animal kingdom can be 

 assigned to them. 



Here, then, we find another instance of a pro- 

 found modification of structure associated with 

 the surface-swimming habit ; the modification 

 being due very largely to the absorption of con- 

 siderable quantities of water into the tissues of 

 the body, which has the effect of rendering them 

 transparent, and, at the same time, of reducing 

 their weight in the water. 



The transparency of the body of so many of 

 the animals of the Plankton has suggested the 

 theory that by rendering them less conspicuous 

 to their enemies it is of the nature of a protection 

 to them. We ought to hesitate before accepting 

 this theory until we know more accurately what 

 are the enemies that they endeavour to protect 

 themselves against. It is very probable that 

 none of the Fish will feed upon any of the trans- 

 parent Jelly-fish, neither is there any evidence 

 that the Salps and the pelagic Gastropods form 

 a favourite food for them. There is no good 

 reason for supposing that the Sea-birds would, if 



