334 



iinfaYOi'al)le bottom, or if food sup])!}" is scarce, thev will 

 perish. Thus, other things l)eiiig- favorable, wherever the 

 facies of sea bottom normal to a particular species of ben- 

 thouic organism exists, that bottom may be ])eopled with 

 that species by the larv^E which reach it from the upper 

 ^^•aters, ^A'here they are carried by waves and currents dur- 

 ing their mero-planktonic Avanderings. As Walther says, 

 should unfavorable circumstances temporarily destroy a 

 whole fauna, its depopulated home will at once be sur- 

 rounded by swarms of delicate larvae, and as soon as the old 

 conditions are reestablished, this fauna Avill again appear 

 with countless individuals. This explains the sudden reap- 

 pearance, in later strata, of the fauna of an earlier bed, even 

 though absent from the intervening strata.* 



From a geological ])oint of view, the mero-plankton is of 

 vast importance, for to it are due the wide dispersal and mi- 

 gration of the benthonic organisms, which of all marine 

 organisms are the best indices of the physical conditions of 

 the sea bottom. It isduringthelarval ])eriod that benthonic 

 marine invertebrates undertake their wanderings, and that 

 migration to distant regions occurs.! 



The term pseudo-plankton was introduced by Schiittt 

 for such organisms, which, like the Sargassum, are norm- 

 ally, or in early life, benthonic, but continue their later exist- 

 ence as ])lanktonic organisms. Walther has extended the 

 meaning of the term so as to include those organisms which 

 are carried about by floating objects, to which they are 

 either attached as sedentar}^ benthos or which serves them 

 as a substratum on which they lead a vagrant lienthonic 

 existence. Such organisms are the alga?, hydroids, and 



* An example of this in the Hamilton group of Eighteen Mile Creek is the fauna of the 

 Demissa bed. near the toji nf the Hamilton sliales, wliieh is in many respects the fauna of 

 the Pleurodietyiini lieils and associated shales near the base of the Hamilton shales, the 

 modifications lieing chiefly in the form of additions. (See the author's paper on Faunas of 

 the Hamilton (Jrouji, etc., p. 312.) Other examples are the frequent reciu'rence of beds 

 crowded with Linrlu/iichus niulticostus, and others crowded with Amboccelia unibonata. 

 at intervals separated by thicknesses of greater or less extent, in which they are rare or 

 wanting. 



t The mero-plankton of the fluvial realm belongs in general to the same classes as that 

 of the marine realm. In the terrestrial realm, the mero-plankton is typically represented 

 Ijy the spores and seeds of plants, and perhaps by the spores or larval stages of some lowly 

 aquatic or parasitic animals. 



i Das Pflanzenleben der Hochsee — Plankton expedition, I. 



