57 



tional Museum, and given by him in describing species in the Trans- 

 actions of the Wagner Free Institute of Science. These have been 

 the publications most relied upon in making out depth and geographic 

 ranges for the living forms. The literature available has not always 

 contained the results of the latest discoveries concerning the habitat 

 of recent species. When these discoveries are made known, this 

 table, especially in the columns headed "Geographical Range," may 

 be somewhat modified. 



Concerning Depth. — Of all the conditions of habitat, none is of 

 much greater influence upon a form than depth. From the investi- 

 gations of Verrill and Smith along the southeastern coast of New 

 England, it is seen that each zone of depth, if that word can be so 

 employed, has its peculiar life-forms, which, should they migrate 

 much beyond their proper depth range, would perish. Of course, 

 many forms live equally well in two or more zones, but others seem 

 to be confined to one particular depth zone, especially such forms as 

 live between tides. The main reason that depth is an important fac- 

 tor in the distribution of life-forms is that variations in depth in sea 

 water brings about a variation in temperature, though variation in 

 pressure, too, is not to be underestimated. Again, in shallow bays 

 and estuaries the water is subjected to greater extremes of heat and 

 cold than is the case on the open coast at equal depths, and only cer- 

 tain forms can adapt themselves to these marked extremes. At cer- 

 tain stages of its existence, a species would be killed by a lowering of 

 the temperature of the water by 5° F., or even less. Brooks's experi- 

 ments in Chesapeake Bay showed that a fall of 2° F. in the tempera- 

 ture of the water killed all the swimming larvae of Ostrea virginica. 

 A rise of twice that many degrees, remarks Dall, would probably 

 have only hastened their development. The general rule is that "it 

 is always easier for a cold-water invertebrate to survive in warmer 

 water than it is accustomed to than for one belonging to warm 

 waters to persist when there is a change to a lower temperature." An 

 increase in depth brings about a lowering of the temperature, and 

 this is the ground for the importance attached to depth range. Now 

 a study of the tables shows that most of the species of which record 

 is had belong to the shallow-water zone exclusively, only 30 out of 

 the 179 species listed ranging over fifty fathoms in depth, and some 

 of these very little over the fifty-fathom line. Only thirteen species 

 range deeper than the 100- fathom, line, and these show a preferred 

 range much shallower than that, as the following list clearly shows : 



Pteria colymbus, 10-180, usually shallow; Diplodanta semiaspera, 



