1909] Bitter: Halocynthia johnsoni n.sp. 9] 



linkage between organism and environment may be put in this 

 way: Are there recognizable physical or biological differences 

 of condition within the habitat of the species, and if so are 

 there structural differences in the animals exactly corresponding 

 to these environmental differences? Obviously, it would require 

 an extensive examination of both animals and external conditions 

 to answer these questions fully, but it is also obvious thai the 

 facts for such an examination are in the main accessible. They 

 may be many, they may, some of them, seem trivial, they may be 

 complex, their ascertainment may require much time and labor, 

 but at least they are not clearly and from the outset wholly be- 

 yond the reach of observation. 



A few of the more easily accessible facts in the case I am 

 able to present. In San Diego Bay the animals are obtained most 

 readily from the piles of the extensive wharves constituting the 

 city's main waterfront. Here they live intermingled with great 

 quantities of sponge, hydroids. tubiculous and other annelids, 

 mussels, barnacles, and other ascidians, and may be gathered at 

 any season of the year in almost countless numbers. But they 

 are by no means restricted to this part of the bay. For example, 

 during the summer just past, a number of specimens were se- 

 cured, through Miss Johnson's efforts, from the few remaining 

 piles of an old wharf at National City situated on the bay four 

 miles farther towards its head than are the main city wharves. 

 Since San Diego Bay is long and narrow, and has but a single 

 communication with the sea, it is highly prqbable that two points, 

 one of which is four miles nearer the mouth than the other, will 

 present considerable differences as regards temperature and 

 density of the water, and completeness of change during each 

 ebb and flow of the tide. 



In order to test this, I asked Mr. G. F. McEwen, the physicist 

 of the Biological Station, who has in hand the study of the phys- 

 ical conditions of the water, to make temperature and density 

 determinations of water samples from the several points in the 

 bay at which //. johnsoni occurs. I will here consider only the 

 results for the two points already mentioned. These may be 

 presented in tabular form : 



