4 University of California Publications. [zoology. 



from the Alaska coast, an account of which Professor Nutting 

 ('01), has recently published. 



If there is one thing more than another which the preparation 

 of this paper has brought emphatically to my attention, it is the 

 great necessity for long-continued observations on the growth 

 and development of hydroids under natural and artificial condi- 

 tions. Unfortunately the systematist is forced to rely for 

 his determinations almost exclusively upon skeletal characters, 

 which are peculiarly variable. Size, habit, branching, length 

 of branches and internodes, thickness of perisarc, shape and 

 ornamentation of hydrothecse, kind and amount of annu- 

 lation, number of tentacles on the hydranth and on the medusa at 

 the time of liberation — all these are subject to often perplexing 

 variations. This leaves little doubt that when proper measures 

 are taken for the observation of living animals the number of 

 species will be materially decreased. 



It is mainly through experimentation that the natural condi- 

 tions of life will be analyzed, and it is only by a thorough 

 investigation of the latter that true affinities can be established. 

 Some of the many causes which affect the lives of hydroids 

 directly have been suggested in the course of the paper. In the 

 cases of Sertularia furcata and 8. argentea, gravity seems to be a 

 factor of importance in determining the direction of growth and 

 the position of the hydrotheea 1 on the stem. In Gonothyrcea 

 clarM and Campanularia pacifica, malnutrition, or some other 

 unfavorable condition not yet known, produces a peculiar attenu- 

 ation of branches and perisarc, possibly by directly stimulating 

 the coenosarcal cells to division while inhibiting the action of 

 those with a glandular function. It is equally desirable to know 

 the factors which determine the position of the gonosome in such 

 forms as Obelia commissuralis, Plumularia goodei, and Sertu- 

 larella halecina, in which species the gonangia take the place of 

 hydranths. 



There are other sorts of questions which can receive as yet 

 but doubtful answers, such as the causes of the seasonal distri- 

 bution of some species, e.g., Tubularia crocea, which dies out 

 in San Francisco Bay during the winter months, though it 

 flourishes the year round at San Pedro; here the result is 

 probably referable to changes in temperature. 



