350 



DEPTHS OF THE OCEAN 



that they do so. There is a series of closely related species, 

 for instance, grouped round Ceratiuvi macroceros. Ceratium 

 arcticum is the farthest outpost in the direction of the polar 

 sea, and shows the greatest variation. Its three horns are 

 extremely divergent ; the centre one, which points forward, 

 is slightly bent, and so also are the other two. Near the 

 southern limit of the species there are more and more instances, 

 in a series of transition forms, where the two posterior horns 

 bend forward, till we get to Ceratium longipes, the characteristic 

 form of the Norwegian Sea and North Sea during the first half 



Fig. 246. — Species of Ceratium belonging to the type of C. macroceros, 

 northern species. 



a, C. arcticum; b, C. longipes ; c, C. macroceros ; d, C. infermediitm (-J-). (Jorgensen.) 



of summer. In this case, the posterior horns are bent quite 

 forward, so that their extremities are parallel with the frontal 

 horn. In the Gulf Stream we get C. intei'-niedium, which has a 

 straight frontal horn, like the other members of this type, and 

 all three of its horns are much longer and more slender than 

 those of the two northern species. At the eastern limit, where 

 fresh water from the Baltic and the coasts of North Europe 

 reduces the salinity, and where, too, the high summer temperatures 

 diminish the viscosity of the surface-layers, there is a species 

 with an even better suspension-apparatus, namely C. macroceros 

 (see Fig. 246). Its frontal horn is particularly long and thin, 

 and the posterior horns first bend a little backwards, and then 



