Image d'archive à partir de la page 187 de l'Ingolf-Expedition danois (1918). Le Ingolf-Expedition danishingolfexpe danois0507ingo Année : 1918 i8o HYDROIDA II la mesure de certains secteurs de la distribution horizontale, et nous pouvons tout d'abord s'efforcer d'obtenir une idée plus claire de ce qui est communément connu sous le nom de la région boréale, la définition de ce qui semble être un peu vague. Dans le cas de l'hydraires, l'enregistrement des données sur fournir de précieuses conseils, comme il ressort de ce qui suit. En théorie, la limite nord de la région boréale devrait être tirée que le bound- ary où sud

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Archive image from page 187 of The Danish Ingolf-Expedition (1918). The Danish Ingolf-Expedition danishingolfexpe0507ingo Year: 1918 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' joo m. boom. _._._. 1000 m. .._„ 2000m. Fig. XC. The occurrence of Nemeriesia antennina, Dynamena pttmria, and Thitjaria tJiuja, 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

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