THE EUROPEAN SEAS. 



19 



inhabiting the littoral zone. The shape and size of 

 testacea found in tideways are very considerably 

 influenced by their situation, and it is in such loca- 

 lities we seek with most success for the curious 

 and beautiful sea-jellies (medusce), whose fragile 

 frames seem often to delight in sporting amidst the 

 agitated waters. 



Currents, besides their agency as modifiers of 

 climate, act as means of transport, and, perhaps 

 above all other causes, are influential in deter- 

 mining the diffusion of marine animals and plants, 

 since, through their help, the germs and larval states 

 of numerous creatures which eventually become 

 fixed and stationary may be carried from district 

 to district, and rapidly extended over vast areas. 

 Even fixed creatures, when attached to bodies, such 

 as masses of wood, capable of easy transport, may 

 have their range materially enlarged by the same 

 cause. 



The influence of climate is conspicuously mani- 

 fested in the diminution of the number of genera 

 and species of marine animals in the European 

 seas, as we proceed from south to north ; this de- 

 crease we can scarcely attribute to other cause than 

 the diminution of temperature. In the warm 

 waters of the southern provinces, whether mediter- 

 ranean or oceanic, the variety of types and the 

 abundance of kinds ranged under them are equally 

 multiplied ; in the colder waters of the north, the 

 forms are not so varied, nor are the species so 



