THE DEEP-SEA FAUNA. 165 



owing to the fact that the temperature of the bottom continues 

 upward along the face of the ridges which separate the Carib- 

 bean from the Atlantic. 



While it cannot be denied that the littoral fauna is largely 

 influenced by the fact that its inhabitants live within the limits 

 of the action of the sun, — that is, in the variable limits of 

 temperature (150 fathoms), — it does not follow that light 

 alone, which perhaps does not penetrate even to one hundred 

 fathoms, is the all -important factor regulating distribution. 

 The action of the waves, of the shore currents, and of the 

 tides, is only felt in that belt, and must exert a powerful influ- 

 ence in modifying the habits of many of the littoral animals and 

 plants. 



Species living beyond one hundred fathoms may dwell in total 

 darkness, and be illuminated at times merely by the movements of 

 abyssal fishes through the forests of phosphorescent alcyonarians, 

 and by the feeble light which many of the deep-sea acalephs, 

 echinoderms, and mollusks emit. Perhaps at those depths as 

 beautiful effects may take place as attend the phosphorescence 

 so common in shallow water along a coral reef. The structure 

 of the organs of vision of the deep-water echinoderms, Crus- 

 tacea, polyps, mollusks, and annelids, differs only in a few cases 

 from that of their congeners in shallower regions, to which the 

 direct light of the sun penetrates. Were it not for the phos- 

 phorescence, we might almost imagine the deej)-sea animals 

 devourino; each other without ever having' seen their food, and 

 living for ages under conditions subject to but trifling changes, 

 •while during the same length of time terrestrial modifications of 

 great importance were taking place. 



Many of the deep-sea gasteropods, as well as some of the 

 fishes and Crustacea, are blind. They belong generally to the 

 more antique types, and the absence of eyes and the presence 

 of rudimentary organs of sense are compensated by the de- 

 velopment both in fishes and Crustacea of tactile organs of 

 gigantic size which serve as special organs of sense. ^ 



^ Grimm, in studying the deep-water oped organs of sight, at the same depths 

 famia of the Caspian, noticed that, while there were genera in which the eyes are 

 several of the Crustacea have well-devel- atrophied, and other organs of sense re- 



