DISTRIBUTION IN SPACE. 155 



DISTRIBUTION. 



1. DlSTRIBUTlOX IN SPACE. 



The data from which an adequate knowledge of tlie Geographical Distribution of the 

 IIydroida may be derived arc as yet very imperfect, and our assertions regarding the horizontal 

 and vertical extension of the group in the present seas of the globe cannot be regarded as 

 possessing more than a provisional value, liable to modifications as additional facts come to our 

 knowledge ; while — if we except the graptolites, whose hydroid relations will be presently dis- 

 cussed — we know almost nothing of the geographical distribution of hydroids in former periods 

 of the earth's history. Indeed, the localities which have yielded the very few fossil hydroids — 

 not being graptolites — hitherto discovered cannot be viewed as representing, even approximately, 

 the former distribution in space of the species which have thus come down to us, and the following 

 remarks on the distribution of the IIydroida in space are therefore confined to the existing forms 

 of the order. 



o. Horizontal Distribution. 



In a complete exposition of the horizontal distribution of the IIydroida our statements 

 ought to embrace not only the fixed elements of the hydrosoma, but also tlie free planoblasts. It 

 is true that a large number of planoblasts still remain untraced to tlieir hydriforni trophosomes, 

 but our want of knowledge in this respect can scarcely afford grounds for rejecting the medusa 

 from our general survey of hydroid distribution, more especially when we bear in mind the 

 probability that the planoblasts, notwithstanding the pelagic habits of these free-swimming buds, 

 never wander far from the rooted trophosomes in which they originate ; and it is only because 

 our positive knowledge of the distribution of the planoblasts is so very deficient that our assertions 

 regarding hydroid distribution must be understood as applying chiefly to the rooted trophosome. 



But even though we take all the facts which have come to our knowledge regarding the dis- 

 tribution of both trophosome and planoblast, we are still far from possessing the data necessary for 

 a satisfactory determination of the geographical distribution of the Hydroida. In a great many 

 important geographical areas we know absolutely nothing of the state of hydroid life, while from 

 many others the facts which have come to us are so few that they are quite inadequate as the 

 basis of a generalisation. 



It is upon the coasts of the British Isles that the most numerous and complete observations 

 have been made, and next to these in importance must be mentioned the north-west shores of 

 the continent of Europe, the Atlantic shores of North America, and the ]\Iediterranean Sea ; while 

 others of considerable value have come to us from the Pacific shores of North America, and from 

 Australia, New Zealand, and South Africa. 



Several hydroid trophosomes have been recently obtained from deep dredgings in the 

 western parts of the Gulf Stream, while dredgings at great depths in the more eastern areas 



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