THE PHYSICAL CONDITIONS OF THE ABYSS 35 



importance ; for, in the consideration of the characters 

 presented by the fauna of any particular region, it is 

 always necessary to take into account the physical 

 difficulties the animals have to contend against and 

 the modifications of structure they present to combat 

 these difficulties. 



Thus in a region such as that presented by the 

 deep sea, where there are no rapid tides, we should 

 not expect to find such a powerful set of body 

 muscles in the free- swimming forms nor such a firm 

 vertebral column as in the animals that live in 

 more lively water. 



Perhaps it is of the nature of an assumption to 

 say that there are no rapid currents and tides in the 

 abysmal depths of the ocean, for we have no means of 

 demonstrating or even of calculating the rate of flow of 

 these waters. But it is a reasonable hypothesis and 

 one that we may well use until the contrar}^ is proved. 



A fact of some importance that supports this 

 hypothesis, as regards some parts of the ocean at 

 least, is presented by the sea-anemones. 



Many of the shallow-water Actinians are known 

 to possess minute slits in the tentacles and disc, 

 affording a free communication between the general 

 body cavity or coelenteron and the exterior. 



