544 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



forms. This brings up the whole subject of the different 

 conditions to which the planktonic flora and fauna are 

 subjected in oceanic and coastal regions. 



It is an important point, because though the oceanic 

 regions are of very great extent, the waters that are of 

 practical importance for fisheries are our coastal seas, like 

 the North Sea, the Irish Sea, &c, where the depth is 

 nowhere very great, but where the plankton is very 

 abundant, and where a thorough planktonic investigation 

 should be of considerable economic value. The bottom 

 of these seas and all coasts is inhabited by a large and 

 varied animal and vegetable population ; the laminarian 

 zone, for example, is probably the richest area of the 

 earth's surface in animal life. From the Echinoderms, 

 the Crustacea, the fish, &c, found in these shallow seas 

 arise myriads of larval forms, which, after a pelagic life, 

 again migrate to the bottom and continue their existence 

 as fixed or sedentary animals. Thus the plankton of the 

 Irish Sea is made up to a very large extent of eggs and 

 larvae of animals which are not pelagic when adult. In 

 one group the Crustacea, for example, there are orders 

 like the Copepoda, which are typical plankton forms and 

 remain, with few exceptions, free-swimming and pelagic 

 during their whole life ; while the Cirripedia, on the 

 contrary, have the pelagic larval forms, but their adults 

 are fixed, and therefore not constituents of the plankton. 



Then, again, the Hydrozoa contain forms which 

 alternate between a fixed hydroid generation and the free 

 medusoid of the plankton. Certain Nereis species are to be 

 found creeping about the bottom or swimming sluggishly, 

 but when sexually mature undergo a considerable change 

 in structure, the parapodia become modified for swimming, 

 and the so-called Heteronereis stage may then be 

 caught in considerable numbers in the plankton nets on 



