334 



unfavorable bottom, or it' food supply is scarce, they will 

 perish. Thus, other things being favorable, wherever the 

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

 thonic organism exists, that bottom maybe peopled with 

 that species by the larvae which reach it from the upper 

 waters, where they are carried by waves and currents dur- 

 ing their mero-planktonic wanderings. 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 will 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 point 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 is during the larval period that benthonic 

 marine invertebrates undertake their wanderings, and that 

 migration to distant regions occurs. v 



The term pseudo-plankton was introduced by Schiittl 

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

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

 ence as planktonic 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 sedentarv benthos or which serves them 

 as a substratum on which they lead a vagrant benthonic 

 existence. Such organisms are the algae, hydroids. and 



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

 Demises lied, near the top of the Hamilton shales, which is in many respects the fauna of 

 the Pleurodictyum beds and associated shales near the base of the Hamilton shales, the 

 modifications being chiefiy in the form of additions. (See the author's paper on Faunas of 

 th>- Hamilton Group, etc., p. 312.) Other examples an- the frequent recurrence of beds 

 crowded with Liorhynchus mvlticostus, and others crowded with Ambocoelia umbonata. 

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

 wanting. 



+ 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 

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

 aquatic or parasitic animals. 



J Das Pflanzenleben der Hochsee — Plankton expedition, I. 



