SHALLOW-WATER FAUNA. 45 



among the most voracious and destructive inhabi- 

 tants of the shallow waters, and it is probable 

 that the covering of spines which we find so 

 commonly among shallow-water animals is an 

 adaptation to prevent or render difficult the 

 operations of these creatures. 



The Gastropod Molluscs form another large and 

 important group of creeping animals of rocky 

 coasts. On nearly all our own coasts numerous 

 Periwinkles may be seen clinging to the rocks at 

 low tides, and if a search be made in the deeper 

 pools and on the rocks nearest to the low-water 

 marks many other species will be found of ani- 

 mals with spirally-coiled shells which are included 

 in this group of Gastropods, The Periwinkles 

 on the rocks might at first sight be thought to 

 belong to the sedentary group of animals, but 

 when the water covers them again, or when they 

 are put into an aquarium, they may be seen to 

 protrude a head and an elongated slimy foot, 

 which, gliding over the surface of the rock, drags 

 the great shell and its contents with it. On the 

 approach of danger the foot and head are with- 

 drawn into the shell, and the animal rests secure 

 from many enemies that might otherwise have 

 found it a dainty morsel. Some of the Gastro- 

 pods are purely vegetable-feeders, but most of 

 those living in shallow sea -water feed upon 

 Molluscs and other animals. It might well be 

 a matter for wonderment when the soft head and 

 little mouth of a Gastropod, such as a Whelk, 

 are examined, that it is carnivorous and attacks 

 and devours animals as large as itself. But the 

 anatomist shows us that hidden in the recesses 



