331 



this group have powei" to propel tliemselves through the 

 water, they nevertheless are subject to the force of strong- 

 waves or currents, which will render them helpless. The ma- 

 rine, or halo-planktonic organisms, are wholly pelagic, and 

 are characterized by a more or less transparent body, and by 

 the absence of opaque skeletal structures, only a few forms 

 retaining delicate calcareous shells inherited from their ben- 

 thonic ancestors. In their horizontal distribution the halo- 

 planktonic organisms ai'e dependent chiefly upon the marine 

 currents, as they are practically unable to undertake inde- 

 pendent migrations, though many of them can dart about 

 in quiet water. They hence fall an easy prey to actively 

 preclaceous animals. The occurrence of these animals in 

 swarms is also accounted for by their lack of independent 

 locomotion, for the eggs, lil)erated by the floating parent, 

 commonly develop without separating far from the parent, 

 with whom 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 hours of the day many of them 

 live at a depth of from fifty to one hundred and fifty fath- 

 oms, coming to the surface only on quiet nights. The ani- 

 mals of this class also occur in the abysso-pelagic district.* 



The halo-plankton forms one of the chief sources of food 

 for many marine animals, and is commonly devoured in vast 

 quantities. The dead organisms which sink to the sea floor 

 in an incomplete state of decom})ositiou form the chief ele- 

 ment of the organic oozes, which farnish food to many 

 littoral, as well as abyssal, animals. The skeletal portions of 

 the dead plankton will often accunuilate in vast quantities 

 on the bottom ; and in the greater depths, where terrigenous 

 sediments are absent, they commonly form diatomaceous, 

 radiolarian, globigerina, pteropod and other oozes. The 

 purity of such oozes, i. e., their freedom from clastic sedi- 

 ment, is usually an index of the purity of the water in which 

 they were deposited, but from this we cannot always decide 



* Fresh-water plankton, or limno-plankton, is to be met with in almost all waters. Prob- 

 ably no permanent aerial or geo-plankton exists, though baeteria and other micro-organisms 

 may float about in the atmosphere for an indefinite period of time. 



