vol. l.] Torrey. — Hydroida of the Pacific Coast. 5 



It is important to discover the relative variability of species, 

 their plasticity or adaptability, what characters are affected 



directly by the conditions of the environment, what arc more 

 stable or not affected at all. So I have tried to bring into this 

 paper as much pertinent ecological material as possible. Most 

 of the western species have been described from preserved 

 specimens, and in many cases there are no records of environ- 

 mental features, such as depth, temperature, character of the 

 bottom, etc. The depth and temperature, whenever known, are 

 given in the table of distribution. The records are necessarily 

 incomplete, and form a rather insecure basis for generalizations 

 at present. 



The development of some species, especially with reference 

 to the appearance of the tentacles, has been briefly considered, 

 being of much taxonomic importance. In connection with some 

 species, especially Gorymorpha palma, I have described certain 

 activities, phenomena of orientation, and processes of regenera- 

 tion, some of the points which have appeared to me to be of 

 general interest. 



Distribution . Of the 140 species on the western coast of 

 North America, 54 (39%) are restricted to this region. Of the 

 remaining 85 species, 24 (17%)* are found on the eastern coast 

 of North America, 11 (8%) in Greenland, 36 (25.7%) in Asia, 

 .3 (3.6%) in South Africa, and 6 (4.3" u ) in New Zealand and 

 Australia. The presence of 11 of the foreign species is recorded 

 for the first time. 



The gi-eat center of distribution is northern. Hdlicoritaria 

 producta is a single exception to the rule, having been collected 

 in Australia and San Diego, California, only. The South African 

 species are circumboreal. Three of the New Zealand species are 

 circumboreal, the other two probably so, since they are found 

 also in Great Britain. Of the foreign species, 4 do not pass 

 the Aleutian Islands from the north; 14 do not pass Sitka, 

 Alaska; 15 reach, but do not pass Puget Sound; 13 are found 

 in Southern California, 4 being peculiar to that region. The 

 northern character of the hydroid fauna is shown by the tables, 



" The percentages are based on the total number. 



