i894. TEMPERATURE AND MARINE ANIMALS. 281 



in the north district, the Alciopidae are wanting there, and occur 

 only south of the Gulf Stream. For other groups, such as Appendicu- 

 laria and Decapods hitherto worked out, the same sharp Hmit has 

 been found. Most sharply defined limits are seen in the interesting 

 pelagic copepod genus Copilia, which is wanting in the north 

 district, but occurs not only abundantly but in many species in the 

 equatorial part. 



It is remarkable that, in the eastern district near Ascension, other 

 species than those occurring in the same latitude near Brazil have 

 been found. I have stated a similar fact for some Medusae. 



The explanation of this is rather simple in connection with the 

 influence of temperature. Here a southern colder stream passes ; 

 stenothermal animals which are adapted to colder water find here a 

 temperature that suits them, while they cannot grow and flourish in 

 the tropical waters near Brazil. The influence of this same cold 

 current has been stated to exist by Studer and by the " Challenger." 

 The family of Geryonid Medusae in general, to which the genus 

 Liriope belongs, are a very good example for the influence of tem- 

 perature on Plankton animals, as Corals are for the Benthos. The 

 Geryonids seem to require warm water and occur only at a certain 

 latitude. They are abundant in the Mediterranean, but are wanting 

 totally in the German Sea and the north district in general. They 

 are abundant in the equatorial part of the Atlantic, and the same 

 may be said of the Pacific. I have obtained them in great number 

 from the warmer currents, which the " Albatross " expedition 

 encountered. I also got them from the Red Sea and the Indian 

 Ocean, but none from higher southern latitudes. 



It is in harmony with this evident need of warmth that the 

 Geryonidae are typical animals of the surface. This is obvious from 

 the so-called " Stufenfange," i.e., captures made at the same station 

 in 50, 100, 200 m. depth ; and they occurred even in the cylindrical 

 surface-net of the Plankton expedition, where many other animals 

 which had been found in 100 m., etc., were wanting. It is known by 

 direct observation how their umbrellas project above the surface of 

 the water, especially during contraction, so that ItaHan fishermen call 

 them " capelletti di mare," or " little sea-caps." 



From Agassiz's last three cruises there is the same result. A 

 Liriope was the most abundant Medusa, always appearing at the 

 surface, even when Stomobvachium and other species had retreated to 

 a depth of 100-200 m., to avoid the influence of daylight and warm sun. 



Here we arrive at the third and last point, the distinction between 

 vertical and horizontal differences of temperature, between which a 

 parallel has been drawn hitherto, but upon insufficient grounds. Down 

 to the depth of 400 m. it is allowable to make a comparison between 

 the changes in the life-conditions, especially temperature in different 

 depths and the changes of the temperature of the surface in different 

 latitudes ; but in greater depth this becomes impossible, the light, the 



