28o NATURAL SCIENCE. Oct.. 



in different latitudes, ought to regulate the distribution of real pelagic 

 life ; a system of belts would result parallel to the degrees of latitude; 

 but this system of parallelism becomes disturbed by the action of 

 currents and winds. On the other hand, by these currents sharper 

 regions are defined ; for, otherwise, all these zones would pass into 

 each other gradually, which is not the case in reality. 



Both north and south of the equator three circles of currents may 

 be distinguished from o-to, 10-50, 50-80° of latitude, and between 

 them islands, as it were, of quiet water are formed, the so-called 

 Halistases. These circles, i.e., depending upon the temperature in 

 them, constitute the most important factor in the distribution of 

 pelagic life. 



A current always shows differences of temperature at its outer 

 limit, and by this circular arrangement the pelagic animals, as Brandt 

 first laid stress upon, are kept within temperatures of a fairly constant 

 degree ; being always carried back to the circle-current of the 

 temperature, and not being able to escape and flourish in the adjacent 

 colder or much warmer water. This, as I wish to emphasise, is valid 

 for the stenothermal animals. Eurythermal animals could be carried 

 away to other regions, and continue to flourish there. Currents, 

 therefore, act in two manners, either as helps to wandering, or as 

 barriers to spreading.^ In general, the latter will be the case for 

 Plankton animals. 



A division of the open sea into zones can, therefore, be 

 accomplished. How great the difference can be on each side of a 

 current may be seen chiefly at the Gulf — or Florida — Stream, as is 

 illustrated by some instances noted by the Plankton Expedition. 



While working out the Craspedota of this expedition, I could as- 

 certain that no pelagic species of Medusa, occurring to the north of the 

 Gulf Stream, is found also to the south of it, and vice versa. The well- 

 known Aglantha digitalis (described first by Forbes) has been caught 

 constantly and in abundance from North England to Greenland. To 

 the south of the Gulf Stream it ceases at once to occur. The same may 

 be said of a Narcomedusa, S<?/;«rtm mnltilobata. On the other hand, 

 Aglaura hemistoma, which has not been found in the north district, 

 occurs with great regularity in the middle and equatorial part of the 

 Atlantic, and the same can be said for the well-known Rhopalonema 

 velatuvi and some species of Liviope. 



Similar observations have been made in other groups of animals, 

 for instance, in the pelagic annelids. The Tomopteridae are abundant 



2 The occasionally abundant occurrence of animals generally rare or totally 

 wanting at the British coast, can thus be explained. The animals in question are 

 stenothermal animals of the Plankton. In ordinary years the Gulf Stream forms a 

 sharp limit for them. If they are driven in the colder water by the current, they 

 cannot grow there, but perish. Now, such uncommon occurrences have been recorded 

 in years of uncommonly warm weather. Then the animals carried away from the 

 south find a suitable temperature at the surface in the north, too, and the action of 

 the current in such cases is dispersing. 



