46 THE SEAS 



Another is furnished by a tropical crab [Melia tessellaia), 

 which carries anemones in each of its claws, first deliberately 

 removing them from the rocks. It holds them fully 

 expanded, pushing them forward when attacked and taking 

 out the food they capture and transferring it to its own 

 mouth. The claws are used exclusively for carrying the 

 anemones. The advantage to the crab is obvious while the 

 anemone, in spite of losing so much of its food, perhaps 

 gains, as by being moved about in this manner it has so 

 many more opportunities of obtaining food. 



Adaptations for Breathing 



The problem of respiration — of obtaining that essential 

 minimum of oxygen, without which no animal can live — 

 is often serious on the shore, where the inhabitants are part 

 of the time in water and the remainder in air. We have 

 not space here to discuss the different organs, or gills, used 

 in different animals for obtaining oxygen, but some instances 

 of shore animals which are able to respire both in air and 

 in water will be of interest. This can be done, in a sense, by 

 a variety of animals which are able to keep their gills moist 

 for considerable periods while out of water, for example, the 

 Crustacea, which have gills covered over by the edge of the 

 shell — as in the higher forms like the lobster and crabs — or 

 in the form of plates beneath the body. Of the different 

 periwinkles, those which live nearest the shore and are 

 often out of water for long periods, have developed the 

 power of breathing air, the walls of the gill cavity having 

 become richly laden with fine blood vessels whereby the 

 blood is able to take up its oxygen from the air. There are 

 crabs which live exclusively on land, some, such as the 

 tropical robber crab, only going to the sea to breed, and 

 having developed a true lung for breathing, while others 

 which have retained their gills have to go down to the sea 



