i8o 



HYDROIDA II 



the extent of certain horizontal areas of distribution, and we may first of all endeavour to obtain a 

 clearer idea as to what is commonly known as the boreal region, the definition of which at times 

 appears to be somewhat vague. In the case of the hydroids, the data on record furnish valuable 

 hints, as will be seen from the following. 



Theoretically speaking, the northern limit of the boreal region should be drawn as the bound- 

 ary where southern species of general occurrence in northern waters cease their northward progress, 

 while similarly, the southern limit should mark the extreme margin of the arctic-boreal species' distri- 

 bution. It should, moreover, constantly be borne in mind that the boreal area is decidedly a "mixed" 



200 m. boom. _._.iooom. 2000m. 



Fig. XC. The occurrence of Nemertesia antennina, Dynamena pumila, and Thujaria thuja, three species which 



are common in more southern European waters, but which only have been found in high arctic waters quite 



exceptionally. (The red line approximately indicates the limit of the cold area). 



area; strictly boreal species are extremely few in number, among hydroids as among other animal 

 groups, and will in many places penetrate out beyond the limits of the narrower region. — As ex- 

 amples of southern species extending up to the frontier of the arctic areas we may take for instance 

 Nemertesia antennina, Dynamena pumila, and Thujaria thuja. These three are practically never found 

 in true arctic waters; where finds have been made, it was always at places where north-going cur- 

 rents render the limits uncertain, or where the varying conditions of the coastal water afford southern 

 species a precarious refuge in some isolated spot within the arctic seas. At such places we may en- 

 counter, for instance, scattered colonies of Dynamena pumila. As a general rule, we find (fig. XC) 

 that these species do not cross beyond the limit of the cold area as indicated by the course of the o° 

 isotherm. At the same time it should be noted that the mentioned species are by no means infre- 



