1914] Esierly: CtenopJiora of the San Diego Region 37 



Since we have found that certain temperatures and salinities are sup- 

 posedly favorable, it might be suspected that periodicity of appear- 

 ance at the surface is in some way connected with these factors, but our 

 field data are inadequate for determining this point. It is known, 

 however, that the temperatures and salinities at which larger catches 

 are made do not prevail during the periods of greatest abundance. 



Pleurohrachia is more abundant by far in August than in any 

 other month, and it is mainly due to the results of August collecting 

 that large numbers of animals are set down under certain tempera- 

 tures and salinities almost to the exclusion of others. With this in 

 mind, it may be stated that during the "summer" (June to October) 

 Pleurohrachia is more abundant above 18° than below; it is especially 

 abundant above 19°, but temperatures above 20° are apparently not 

 so favorable as those between 19° and 20°. Furthermore, salinities 

 ranging from 33.61 to 33.75 are optimum during the summer but 

 more animals were taken between 33.71 and 33.75 than at any other 

 points. 



As was indicated in the course of the paper it does not appear 

 why August should be the best season. That month, as shown in 

 table 3, is characterized by high temperatures in the vicinity of San 

 Diego, but the same temperatures in other months (October, for ex- 

 ample) are not those at which the organisms are most abundant. 

 What other factors enter into this is largely a matter of conjecture 

 at present. We know that the ctenophore is obtained during the win- 

 ter in fairly large numbers when the water ranges from 14° to lfi° 

 in temperature. Some of the animals that are taken under those con- 

 ditions are characterized by their large size ; they may reach a height 

 of 12 or 15 mm., while during the summer all the animals are small, 

 those 9 mm. high being of rare occurrence. The large specimens have 

 been taken during August in the region around Santa Monica, where 

 the temperature of the hauls averaged 16°-17°. The winter collections 

 and those in summer in the colder water both contain animals as small 

 as anj^ taken in warm water, but large animals do not appear except 

 in the collections from cold water. The fact that we get large animals 

 during August if the water is about 16°, as well as during the winter, 

 seems to indicate strongly that the matter of seasonal distribution is 

 connected with the time of the appearance of the large and presum- 

 ably mature animals. 



The principal aim of this paper has been to set forth as far as pos- 

 sible, on the basis of our field data, something about the manner of 



