Worms belonging to the groups called Polychaetes and Chae tognatha (these are the bristly jointed worms and the translucent "arrow-worms") abound. The only planktonic vertebrate (or rather "chordate") animals are some of the Tunicates, and one kind of these creatures—the salps—may at times be so very abundant in the sea as to clog and sink the drift nets set by the herring fishermen. The Heteropods and Pteropods are the only permanent plankton dwellers belonging to the Mollusca. There are no Echinoderms that are always planktonic but the beautiful larvae of these animals live for a time pelagically. The Coelen terates are, some of them, permanently planktonic (see above), but most of them (the zoophytes, sea anemones, corals, etc.) live on the sea bottom and their larvae appear for a short time only among the plankton. None of the sponges are permanent plankton denizens.
Great numbers of species of unicellular animals and plants are always planktonic : we shall notice the principal kinds very briefly, premising that their full description means writing great books on zoology and botany. Among the Protozoa are the Foraminifera and Radiolaria—the former characterized by their calcareous and the latter by their siliceous skeletons. These animals are micro scopic in size (though sometimes big enough just to be discerned by the naked eye). They are found everywhere in the sea and they are so numerous that extensive regions of ocean-floor, at depths of about 2 to 3m., are covered with fine oozes made up almost entirely of the shells of Foraminifera and Radiolaria that live and die in the upper layers of the ocean. These shells are beautiful and interesting objects when seen under the microscope. Very important from the human point of view are the Peridini ans : these are unicellular organisms that can be regarded either as plants or animals and they are about as abundant as the Pro tozoa that we have already mentioned. They also have very ele gant shells made of a kind of cellulose material which is impreg nated with silica. Great numbers of Peridinians live in the sand and mud in the very shallow sea bottom near the land, but most of the species are pelagic—that is, they are permanently plank tonic. The pelagic Peridinians are important food materials for some fishes—mainly the pilchard (when it is in the sardine stage). These organisms are interesting from another aspect— it is they that mainly cause the phosphorescence of the sea. Sometimes, when the sea is very fiery, an organism called N octi luca is so abundant that the water may be visibly turbid and discoloured.
The Diatoms are to be regarded as unicellular, algal plants, and they are, on the whole, the most abundant of all the planktonic organisms. They have external, transparent, siliceous shells which are elegant in form and are often sculptured with beauti ful patterns. They exist everywhere in water—in ponds, lakes,
on the mud and sand of the foreshore and sea bottom, attached to weeds, etc.—the main restriction on their presence is the absence of light, for, being green plants, they cannot live except in the presence of solar radiation. There are innumerable species and they are so abundant in the ocean that millions of square miles of bottom, at about 2,000 fathoms in depth (and mainly in the Antarctic) are covered by the dead shells of Diatoms.
The Seasonal Variations in the Plankton.—Everywhere in the sea and in fresh waters the composition and abundance of the plankton change with the season. To some extent the nature and sequences of these changes depend on- the region which is being investigated and this is particularly the case when we con sider the larvae, for the nature and abundance of these will de pend on the nature of the local benthic and nektic faunas. If, however, we consider the North Atlantic region the following general descriptions will apply, to some extent, at all events, to most local regions.
The cold and dark months, December to February, are generally periods when the marine plankton is scarce and when the Diatoms and Peridinians, in particular, are relatively few in number. This is because of the feeble intensity of sunlight, which is necessary for the growth and reproduction of these organisms. But some time about March or April there is a great outburst of life in the sea. Certain indispensable food materials (inorganic nitrogen compounds, silica and phosphates) have been accumulating dur ing the dark months; in the spring there is a rise in sea tern perature and this acts as a stimulus to vital processes, but the main factor is the increase in the length of the day and the grow ing intensity of sunlight. The result of these changes is that all the vegetable plankton, but particularly the Diatoms, become stimulated to reproduce, so that there is almost an explosive out burst of life in the sea and great swarms of Diatoms invade the shallow sea areas. This is the great event of the year—the "ver nal maximum" of vegetable life in the sea.