30 University of California Pnhlications in Zoology [Vol. 13 



It is plain from this table that the average number of animals ob- 

 tained per hour is highest when the temperature ranges from 19° to 20° 

 (line 4), and that the next highest hourly average is at temperatures 

 of 20?1 or more (line 5). The numbers of animals obtained at tem- 

 peratures of 18° or less are practically negligible, and the frequencies 

 are very low, while between 18?1 and 19° the hourly average and the 

 frequencies show a marked advance toward the maxima. That is, the 

 abundance is greatly increased after the temperature rises above 18°, 

 and becomes overwhelmingly greater above 19°. 



Tables 5 and 6 emphasize the manner of distribution of the Pacific 

 form of Pleurohrachia, both as to season and temperature. Whether 

 the organisms referred to by the names pileus and hachei are actually 

 of the same kinds or not, they are certainly much alike in structure. 

 Their unlike behavior, however, may be an index to quite dissimilar 

 constitutions not revealed by the visible makeup. In the Atlantic the 

 ctenophore is more abundant when the temperatures are lower ; in the 

 Pacific the reverse appears to be the case. But since season and tem- 

 peratures are associated in characteristic ways, it is hard to distin- 

 guish between their efi^ects. As already mentioned, temperature does 

 not alone bring abundance of ctenophores, because we have records of 

 numbers of hauls, at as high temperatures as those of August, in 

 which the animals were of very infrequent occurrence. 



Pleurohrachia occurs in this region in relatively small numbera 

 during the winter when the water is much colder than in the summer, 

 but, as has been stated, the proportion of large animals is much greater 

 in the winter; in fact, it is not known that specimens over 9 mm. in 

 height are fotind in M^ater above 17°. If such temperatures are found 

 during the summer the large animals are obtained, as already stated 

 in the account of the collections made in the vicinity of Santa Monica. 

 Table 7, dealing with winter months, is similar to table 5 for the sum- 

 mer months. 



TABLE 7 

 Total Number of Hauls, Number of Successful Hauls, Numbers of Pleuro- 

 hrachia, IN Hauls During Each of the Winter Months at Temperatures 

 Up to 19°. Left-Hand Figures, Total Hauls; Center Figures, Successful 

 Hauls; Eight-Hand Figures, Numbers op Animals. 



Temperatures 



14° and less 



14?1-15° 



15?1-16° 



16?1-17° 



17?1-18° 



18?1-19° 



