228 THE SEA FISHERIES 



minute organisms of the plankton are retained by the gill-rakers, 

 whence they are washed off by the fish's tongue and swallowed. 



The constituents of the plankton are now well known, the 

 organisms have been identified and described, and of recent years 

 more or less successful attempts have been made to determine by 

 quantitative methods and chemical analysis the productivity of 

 the sea and the food value of the planktonic material. Nearly aU 

 forms of marine life are represented in the plankton at some stage 

 or other of their life history. Fish are only fouiid in their young 

 arid helpless stages as eggs or laryse. Later in life, fish belong to 

 the Nekton. Crustacea form one of the most imjportant groups of 

 the plankton, some of them being represented in both their young 

 and adult stage?, others in the larval form only. There are huge 

 swarms of small Crustacea known as Copepoda (lit. oar-footed), 

 which are the niost abundant of. all animals in the sea. These 

 creatures form the main source -of the food of pelagic fish, such as 

 the herring. Crabs and lobsters are represented in the plankton 

 in their larval' free-swimming stages only, during which a remark- 

 able developmental metamorphosis is undergone. The adult 

 settles down on the bottom and then ranks among the Benthos. 



Molluscs only appear in the plankton in their larval period ; a 

 statement which is only strictly true as regards edible species. 

 There are large groups of molluscs which are planktonic animals 

 tor the whole of their lives, e.g. the Heteropoda and Reropoda, 

 "biit oysters, mussels, cockles ethpc genus omne belong during their 

 adult' existence to the littoral Benthos. These edible bivalves 

 produce eggs which are feirtilised in the sea water. The eggs while 

 floating about develop into a larva quite unlike the parent . Finally 

 the lairva settles down on 'the bottoin and joins the sedate com- 

 munity of the Benthos. ' 



Marine worms for. the most part are only found in the plankton 

 during their larval existence, but there are a few groups which are 

 entirely pelagic. The starfish and sea-urchin group! (Echinoder- 

 mata) all hve on the sea bottom, but their larvae are found in the 



planTiton. ■ > 



The jelly-fish, medusae and other members of the Gcelenterates 

 also belong to the drifters. Finally there are a' large nuitiber of 

 unicellular forms. Some of these are clearly animal, others no less 

 clearly vegetable, while still others are of doubtful position in the 

 organic world. So numerous are these creatures in the sea that 

 it is no exaggeration to say that every drop of sea water at the 

 surface contains some form of life. The chief animal forms belong 

 either to the Infusoria, the Foraminifera or the Radiolana. The 

 two latter groups are well known, since their shells form the deep- 



