DISTRIBUTION IN SPACE. 159 



colonies, and to whose bottoms the liydroids may adhere. It is interesting, and^ confirmatory of 

 this view, to find the specimens of Scrfulnria abiefina infested in precisely the same way as British 

 ones with SjArorhis cowuiiiiiis and Filellum serpens, both quite chai'actcristic dwellers on the 

 specimens of Sertidaria ahietina which occurs in such abundance round the British shores. 



It is an interesting and significant fact that the distribution of tlie most widely disseminated 

 species is not to be accounted for by any locomotive faculty or pelagic habits of their sexual 

 buds, for the most cosmopolitan hydioids never produce planoblasts at all, their se.\ual l)U(Is 

 being in the form of fixed sporosacs. 



Indeed the hydroid medusit- are nnicli more local in their distribution than what one might 

 at first be led to suspect, and it is highly probable that the planoblasts, notwithstanding their 

 powers of locomotion, never, unless drifted by winds or currents, wander far from their fixed 

 trophosomes. I can find no evidence that any one of the hydroid medusa? enumerated by Forbes 

 in his Monograph of British Naked-eyed Medusae has been met with beyond the province dis- 

 tinguished below as the Boreo-Celtic. Several species of hydroid medusae have been observed 

 by Mertens in the North Pacific and other seas during his voyage round the world, while none of 

 them have been as yet recorded from localities at a distance from those in which this excellent 

 observer had noticed them. A similar localization may be affirmed of the species which Peron 

 and Lesueur as well as other circumnavigatory voyagers have noticed in intertropical and other 

 sens traversed by them ; while those which Gegenbaur has so fully described from the Mediter- 

 ranean appear to be similarly limited in their distribution. 



There can be little doubt that some facts in hydroid distribution must be attributed to the 

 agency of man, and that hydroid trophosomes are not unfrequently carried to a distance while 

 attached to the bottoms of ships and to floating timber. The exceptional correspondence between 

 the hydroid faunas of South Africa and Britain has been just accounted for in this way. It is 

 possible that Cord^Iophora hiciisfris owes its introduction into our docks, canals, and rivers to a 

 migration of this kind, which would have its exact parallel in the case of Dreisse.na polymorpha, 

 a mollusc which had been undoubtedly introduced into this country in a similar way, and 

 is now by no means sparingly distributed through the lirackish and fresh waters of the British 

 Isles. 



What has been just asserted regarding the wide distribution of generic forms must be 

 regarded as referring only to the calyptoblastic hydroids, for if we except the collections made by 

 the North American zoologists on their own shores, it is only the calyptoblastic species which 

 have been collected from distant regions of the globe in sufficient numbers to justify generalisa- 

 tions as to their distribution, or— with very few exceptions — in a sufficiently good state of preser- 

 vation to render possible a satisfactory determination of them. 



Even among the calyptoblastic hydroids we are aluiost confined to the Serfidarince for 

 facts on which to base any reliable conclusions as to distribution. Almost all the specimens of 

 hydroids which have been brought to us from other parts of the world than our own latitudes 

 have either been picked up dead from the seashore, or if obtained in a living state by means of 

 the dredge, have been brought to this country in a dried condition. Now no gymnoblastic hydroid 

 can under such circumstances retain any characters of value ; and even among the calyptoblastic 

 species the Canpanularina, with their delicate and easily detached hydrothecae, are in their dried 

 state often little better fitted than the dried Gymnohlastea for satisfactory determination. Until, 

 therefore, collectors make it a point to secure fresh specimens and put them at once into spirits. 



