152 MARINE ANIMALS 



A considerable number of pelecypods and some snails range from the 

 surface to 2000 and even 4000 m. Thus, among the former, Modiolaria 

 discors is found from to 3250 m., and Scrobicularia longicallus from 

 35 to 4400 m.; among snails, Natica groenlandica ranges from 35 to 

 2350 m. 7 Examples of stenobathic forms of shallow waters are the 

 mollusks Patella, Purpura, Haliotis, and Mytilus, the annelid Areni- 

 cola, and the reef corals (Madreporaria). Stenobathic forms of deep 

 water are represented by the snail Pleurotoma, the mussel Limopsis, 

 and the selachian Chimaera. It is likely that pressure relations have 

 little to do with this limitation. Temperature probably is of greatest 

 importance ; then there is the struggle for existence, and perhaps other 

 causes as yet unknown. 



The internal friction or viscosity of sea water, with its variations 

 due to temperature changes and variation in salt content, and its influ- 

 ence on the suspension of organisms in the water, will be discussed in 

 connection with the pelagic fauna (Chapter XIII). 



Waves, tides, and currents. — The distribution and nature of the 

 marine faunae are influenced in the highest degree by the varied move- 

 ments of the sea water, alike by the waves which break as surf on the 

 beach, the tides due to cosmic causes, and the more or less constant 

 oceanic currents, due to differences in density in different latitudes and 

 to the effect of constant winds. The effects of these movements of the 

 water upon its animal life are in part direct and in part secondary. 



The pounding force of the surf may reach high values. Engineers 

 reckon the average force for the North Sea at 15,000 kg. per sq. m., 

 or 1.5 kg. per sq. cm. Animals exposed to moving water on rocky coasts 

 are thus in danger of being crushed by the water or torn from their 

 places and hurled against the rocks. The inhabitants of rocky coasts 

 within the surf line must therefore have strong protective covering and 

 must be able to attach themselves tightly in various ways. They may 

 be grown fast to the rocks, like the barnacles, or hold themselves by 

 means of a powerful suction apparatus like the foot of the chitons and 

 many snails {Patella, Haliotis), or they may anchor themselves by 

 means of glandular secretions like the byssus of certain mussels 

 {Mytilus, for example) . The shells of the limpet {Patella) are lower, 

 smaller, thicker, and more irregular in outline in exposed places than 

 in sheltered ones. 8 



A Mytilus shell from the wave-beaten west coast of England may 

 weigh 58 g. as compared with 26.5 in one of the same size from the 

 protected Bay of Kiel. 9 Many species of sea urchins on exposed coasts 

 bore holes into the rocks, while the same species in the quieter seas 

 {Strong ylocentrotus in the Mediterranean, for example) does not make 



