202 University of California Publications in Zoology [Vol. 18 



of the habitat of the plankton varies with the seasonal changes. At the 

 beginning of the summer the plankton generally is abundant at a 

 depth of 20 meters. With the increasing surface temperature, how- 

 ever, it gradually sinks to greater depths and in late summer it is 

 found at a depth of 60 to 80 meters (Steuer, 1911). 



Observations on Sagitta hipunctata have revealed the fact that this 

 species increases in abundance as the temperature increases from 

 9° to 14° and decreases as the temperature increases from 16° to 21° 

 (Michael, 1916). This is a clear proof that the maximum tempera- 

 ture for that species lies between 14° and 16° ; hence the species 

 undergoes an oscillation downward or upward with the increase or 

 decrease of surface temperature. 



It has also been observed that some animals, such as medusae, 

 crustaceans and pteropods, come to the surface during the night, and 

 sink to a greater depth during the daytime. A sudden change in 

 temperature may kill the larvae of aquatic animals. Murray and 

 Hjort (1912) have observed that if the eggs of Cucumana are shed 

 in summer when the surface temperature of the Arctic waters is 

 high, they are killed without hatching a single larva. Adult animals, 

 as a rule, can stand more fluctuation of temperature, but there is a 

 maximum and a minimum temperature above or below which the 

 animals cannot live. The power of adaptation differs in different 

 animals and consequently the maximum and minimum temperature 

 cannot be the same for all animals. Hence the species will become 

 adapted to that environment which is best fitted for its existence ; 

 while animals unable to adapt themselves perish. In whatever way 

 temperature affects animals, it is evident that it plays an important 

 role in their distribution by serving as a barrier. 



One of the best illustrations of the effect of temperature is Wyville 

 Thomson's Kidge. This ridge stretches from Iceland to Shetland, 

 separating the Atlantic Ocean from the Norwegian Sea. The tem- 

 perature in the upper strata of water, extending from 400 to 500 

 meters depth, is the same on both sides of the ridge, and the fauna 

 of the upper strata is alike in both regions. But at a depth of 1,000 

 meters the temperature on the Norwegian side is below 0°, while on 

 the Atlantic Ocean side it is 6° to 7°. The deep sea faunas on the 

 opposite sides of the ridge differ greatly. Of 216 species taken from 

 the depths of the warmer region, and the 217 species from the colder 

 region, Murray (1898) found that only 48 species were common to 

 both sides. Other oceanic areas where the temperature changes less 



