HOLOPLANKTON; MEROPLANKTON 993 



course. These organisms range in size from creatures of micro- 

 scopic dimensions to medusas 50 cm. in diameter. While some of 

 the larger animals of this group have power to propel themselves 

 through the water, they nevertheless are subject to the force of 

 strong waves or currents, which render them helpless. Holoplank- 

 tonic organisms are wholly pelagic, and are characterized by a more 

 or less transparent body and by the absence of opaque skeletal struc- 

 tures, only a few forms retaining delicate calcareous shells, inherited 

 from their benthonic ancestors. In its horizontal distribution the 

 holoplankton of the sea is dependent chiefly upon the marine cur- 

 rents, as the organisms composing it are practically unable to carry 

 on independent migrations, though many of them can dart about 

 in quiet water. Hence they fall an easy prey to actively predaceous 

 animals. The occurrence of holoplanktonic animals in swarms is 

 also accounted for by their lack of independent locomotion, for the 

 eggs, liberated by the floating parent, commonly develop without 

 separating far from the parent, with which they are carried along 

 by the currents of the sea. These animals have, however, the 

 power to rise and descend in the water, and during the day many 

 of them live at a depth of from fifty to one hundred and fifty 

 fathoms, coming to the surface only on quiet nights. The animals 

 of this class also occur in the abyssopelagic district. 



2. Meroplankton. This term (from ju.epos, a part) was intro- 

 duced by Haeckel in 1890 (4) and is applicable to the larvae of ben- 

 thonic animals which lead, during the larval stages, a truly plank- 

 tonic existence and which occur with and sufifer the same vicissi- 

 tudes as the true or holoplankton. The upper levels of the ocean 

 are usually crowded with such meroplanktonic organisms, and to 

 them is due the horizontal distribution of benthonic species. Float- 

 ing about in the sea in perfect clouds or swarms these mero- 

 planktonic organisms pass their short existence a sport of the waves 

 and currents. Sooner or later, however, they sink to the bottom, 

 a veritable rain of seedling organisms ; and, if they fall on a fertile 

 soil, if they reach the proper facies of the substratum, they develop 

 into the benthonic adult ; but, if they fall upon an unfavorable bot- 

 tom, or if the food supply is scarce, they perish. Thus, other things 

 being favorable, wherever the facies of sea-bottom normal to a 

 particular species of benthonic organism exists, the bottom may be 

 peopled with that species by the larvas which reach it from the 

 upper waters, v/here they were carried by waves and currents 

 during their meroplanktonic wanderings. As Walther says, should 

 unfavorable circumstances temporarily destroy a whole fauna, its 

 depopulated home would at once be surrounded by swarms of 



