GEOGRAPHIC DIVISIONS OF THE PELAGIAL 275 



decapods, only a few species of shrimps are present in the higher 

 latitudes.* 7 - 8 Hermit crabs and true crabs seem to be entirely absent. 



The polar plankton is distinguished by the great scarcity of free- 

 swimming larvae. It also differs from the warm-water plankton in its 

 distinctive copepods, some of which appear in enormous numbers of 

 individuals, such as the large Calanus hyperboreus (9 mm. long) in 

 the Arctic Sea, the great abundance of which is responsible in a great 

 measure for the wealth of animal life in the Arctic. Less characteristic 

 are the hyperiids and schizopods, and great quantities of various 

 species of pteropod snails among the mollusks. Hydromedusae are 

 much more abundant in cold seas than in warm water. The abundance 

 of diatoms in the polar seas furnishes many plankton animals both 

 in the north and the south with siliceous skeletal material, used, for 

 example, by many radiolarians (species of Aulocleptes) and a number 

 of ciliates (Lamprotintinnus) . 9 



The density of population of the pelagic animal life shows charac- 

 teristic differences. At least in the Atlantic, the cold water is richer in 

 life in general and in animal life in particular than the warm water. 

 Lohmann 10 obtained an average count of 2500 organisms in 1 liter 

 of cold water, and only 700 in warm water above 20°, a fact at least 

 of value as a ratio. Hensen summarizes the results of the investiga- 

 tions to date by saying that the arctic areas are richer in plankton in 

 the summer and the tropical areas are meager throughout the entire 

 year. At any rate, a very definite periodicity exists in the plankton 

 of the polar areas. At the Gauss Station in the Antarctic, the volume 

 of plankton varied in the following proportions during the course of 

 the year: it showed its minimum in winter (June to December), in- 

 creased sevenfold in January, increased to twenty-five fold in Feb- 

 ruary, and reached its high point in March when it was fifty times 

 greater than in winter; decreased in April to twenty times its lowest 

 volume, and returned to minimum volume again in May. In the 

 North Sea, a vertical pull of a net through which 1000 tons of water 

 was seined brought up about 400 Calanus in February, in April, on 

 the other hand, about 4,000,000. n The enormous development in sum- 

 mer is naturally determined in large part by the strength and duration 

 of the light; a thorough light penetration at least of the surface layers 

 for about 20 hours in summer must be extremely favorable for the 

 development of the phytoplankton, and thereby much food is provided 

 for a rich fauna; in winter, on the other hand, plant life is entirely 

 wanting because of lack of light, and only dormant stages survive. 



*In the Arctic, Sclerocrangon ferox, Bythocaris and Hymenodora glatialis; in 

 the Antarctic, Crangon antarcticus and Choriomus antarcticus. 



