HYDROIDA II 179 



A great majority of the northern hydroids are decidedly inhabitants of the littoral region, and 

 among these we find a number of species which characterise its upper part, the tidal zone. This zone 

 comprises, on open sea coasts, the belt between high- and low-water marks; in fjord grounds, how- 

 ever, where the water is mixed with fresh, we are obliged, on account of the altered hydrographical 

 conditions, to draw the lower limit somewhat farther down, where the laminariae begin. The tidal 

 zone is in all parts of the northern waters characterised by Fncoids, and is distinguished in hydrographical 

 respects by its rapidly and widely fluctuating temperature and salinity. Certain writers have objected 

 to the validity of this zone as such, on the ground that all its character organisms are also normally 

 to be met with deeper down; this, however, merely reveals a lack of knowledge as to the true state 

 of the case. We may in the first instance take a species which has been utilised in several works 

 for giving the zone in question its other name; to wit, Ba/anus balanoides Linne. This species does 

 not normally penetrate deeper clown. In this connection, however, we are naturally more interested 

 in the characteristic hydroids; which are: Cory in- p/isilla, Clava multicornis and Laomedca flexnosa. 

 These species are altogether restricted to the tidal zone, and their occurrence in deeper water in the 

 open sea must be regarded as an anomaly, unless due to erroneous identification on the part of the 

 investigator. It is likely that also other species, especially certain Corync, should be grouped among 

 the character forms of the tidal zone, but we have not yet sufficient data to assert this as a fact. The 

 mentioned species are so completely restricted to the area in question, — which is not rich in species 

 on the whole — that we are fully justified in distinguishing the tidal zone as a separate area with a 

 fauna of its own. 



The deeper zones are, from the character of the investigations, somewhat less strictly defined. 

 It would nevertheless seem that species such as Eudendriuin Wrigliti, Sertularella rugosa, and Lao- 

 medea geniculata are altogether restricted to the Laminaria zone, while Dynamena pumila is common 

 to the shallower part of this zone and the tidal zone. The great bulk of the hydroids commence to 

 appear in the lower parts of the laminaria zone, reaching their most luxuriant occurrence in the red 

 alga zone and the lower parts of the littoral region. 



On passing down beyond about 600 metres depth, however, the lower limit of what may be 

 called the "coast bank region", we encounter a sudden and very marked decrease in the number of 

 hydroids; below this limit, their occurrence must be called more sporadic, and there are only a few 

 unique species which have their chief occurrence at greater depths. The deep-sea species in question, 

 which have their habitat down here, exhibit also, in each case, a merely sporadic occurrence, — though 

 we cannot, of course, entirely disregard the possibility that this apparent state of things may in re- 

 ality be due to lack of sufficiently exhaustive investigations in the proper localities. A clear indica- 

 tion of the inadequacy of our knowledge as to these deeper parts of the sea floor, even in the North 

 Atlantic, is afforded by the "Ingolf expedition's unexpected finding of species such as Branchiocerian- 

 t/ius reniformis, Zygophylax biarmata, Grainmaria conferta, Polypi it maria profunda, and Sertularella 

 amphorifera close south of the submarine ridges in Davis Strait, Danmark Strait, and between Ice- 

 land and the Faroe Islands. 



With the paucity of available material, it is of but little interest to go into details regarding 

 the bathymetrical distribution of the species. On the other hand, it is well worth while to consider 



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