iS94. TEMPERATURE AND MARINE ANIMALS. 279 



temperature ; but the case is different with the two remaining 

 biological groups of marine life, the Plankton and the Benthos. 



The difference between the latter is the second point hitherto 

 often neglected in the question of the influence of temperature. 



By the term Benthos is meant any animal which passes its life in 

 connection with the ground, whether it be the bottom of the deep 

 water or the shore of the shallow water. We may distinguish, 

 according to Haeckel, sessile Benthos, i.e., animals totally fixed, and 

 ambulatory Benthos, i.e., animals capable only of creeping and 

 crawling over rocks and sands, but not of swimming great distances 

 at will like the Nekton. 



Plankton is capable of travelling long distances, not at will, but 

 only floating or drifting about with the currents. We might further 

 distinguish a littoral Plankton, the Plankton of the shallow water, the 

 shore, and the Plankton par excellence, that of the High Sea. 



Distinct as these groups are, there may, nevertheless, be 

 transitions between, even in the life of one single species. A Medusa, 

 as fixed at the stem of a Polypoid, is a Benthos animaJ — when set free, 

 an animal of the Plankton. But such cases occur only among the 

 animals of the littoral Plankton, not in the Plankton of the high sea ; 

 there are other Medusae which never have a sessile stage in their life, 

 and are, like Salpae, Radiolaria, etc.. Plankton dimvcysls par excellence. 



These different groups should not be confused with regard to 

 their relations to temperature. Among the animals of the Benthos 

 there are, as I have said, a number of eurythermal ones, as shown by 

 experiment. But the majority are tuned, as it were, to a certain 

 degree of temperature, which, just like the relations to the light, to 

 the character of the bottom, and so on, must be reckoned among the 

 peculiarities of tl^e species. The Benthos animals are mostly 

 stenothermal, and hence the influence of the temperature on them is 

 very remarkable. 



The coasts of the sea might, therefore, be divided, in the first 

 instance, according to latitude or isocrymal lines, into zones, which are 

 broken up into areas by a longitudinal division according to the 

 continents. 



Dana, Giinther, and Agassiz have given us very valuable guidance 

 for the animals of the shore. It is true that, by adding the 

 longitudinal division to the zonary, temperature, which effects the 

 arrangement into zones, does not remain the only factor in the 

 distribution of the Benthos, as the fades of the bottom, etc., are not 

 to be neglected. Still it is regarded as the most important one. 



The distribution of the pelagic life of the open sea is not so clear. 

 The longitudinal division is now without significance, because the 

 influence of the continents does not come into the question. It would, 

 however, be incorrect to think that the true Plankton animals are 

 quite cosmopolitan. 



From the a priori point of view, the warmth of the sun, different 



