1 86 



HYDROIDA II 



consist of a more or less broad belt, and such belts or mixed areas may under special circumstances 

 attain relatively considerable extent, while in other places they may be quite narrow. We must also 

 remember that the boreal region in itself is the mixed area par excellence, and has practically not a 

 single species to itself, which makes it even more difficult to say what mixture percentage should be 

 taken as limiting the extra mixed areas, the boreo-arctic and the boreo-lusitanian. The maintenance 



of these will as a matter of fact be 

 to some extent a matter of arbitrary 

 preference. 



The factors exerting principal in- 

 fluence in the formation of the mixed 

 areas are the positive transportation 

 and the passive restriction of the at- 

 tached organism to the spot once 

 adopted. The latter factor presents no 

 difficulty; if a hydroid colony has once 

 settled down at a given spot on the 

 bottom, then it cannot change its si- 

 tuation, even though the conditions 

 under which it is there called upon to 

 live prove unfavourable. A submarine 

 wave may have produced suitable con- 

 ditions at the time of attachment, af- 

 ter which a change for the worse sets 

 in. If the colony manages to thrive 

 in spite of this, it will then depend 

 on its propagation whether the species 

 becomes indigenous or not. It is pro- 

 bably the question of propagation which 

 has prevented Plumulariidce and Ag- 

 laophcniida- from becoming indigenous 

 in the Norwegian Sea area; save for 

 a few species, they must evidently be 

 constantly recruited from without. This renewal takes place in the case of certain species, which lack 

 suitable active motive apparatus, by passive transportation. The transport of grown colonies is here 

 practically speaking out of the question; these animals, we know, attach themselves normally to the 

 sea bottom itself, or to other colonies already attached thereto. It is therefore larval transportation 

 which plays the chief part 1 . How long the larval development takes in these forms before they at- 

 tach themselves to the bottom we do not know; probably the time varies, being doubtless dependent 

 upon external physical conditions; under certain circumstances it would seem that it may extend over 

 a very considerable period. 



1 Cf. Appellof 1905, Havbundens Dyreliv, Norges Fiskerier I, Norsk Havfiske. Bergen p. 114. 



Fig. XCV. Currents of the Norwegian Sea 

 (after Nans en and Helland-Hansen). 



