188 THE OCEAN 



very uniform, with deposits of soft muds, 

 organic oozes, and red clays. Rocks and 

 stones are only occasionally present. 



The limit between the photic and aphotic 

 zones is deeper at the equator than towards 

 the poles, and is the most interesting region 

 of the ocean because of the change in the 

 physical conditions which there takes place. 

 Sunlight has there its limit of penetration, 

 and in the tropics there is a great change in the 

 temperature. On the bottom currents and 

 waves have little effect, and the organisms 

 exhibit adaptation to these changed conditions 

 in their colours, eyes, phosphorescent organs 

 and tentacular appendages. The mud-line is 

 evidently a great feeding ground, for at this 

 depth the minute organic particles settle on 

 the bottom, and in mid-ocean similar particles 

 are much retarded in their fall by the 

 great increase in the viscosity of the water, 

 thus producing a sort of " artificial bottom " 

 which seems likewise to be a great feeding 

 ground. 



In treating of the forms of life in ocean 

 waters it will be convenient to speak in this 

 chapter of the plants and in the following 

 chapter of the animals. 



Plants in the Ocean. — Phanerogamic plants 

 are represented in the ocean by the family 

 Zosteraceae. Zostera marina (eel-grass) is very 



