CHAPTER 4 



The aiiitiials — the zooplaiiktoii I 



Biologists classify the plant and animal kingdoms into a number of major 

 groups called 'Phyla', each of which is divided into 'Sub-Phyla', then into 

 'Classes', 'Orders', 'Families', 'Genera' and 'Species', with sub-divisions as 

 necessary. For example: Man belongs to the phylum 'Chordata' as do 

 also some very peculiar small planktonic creatures, to the sub-phylum 

 'Vertebrati' as also do the fishes and frogs, and to the class 'Mammalia' 

 as do the mice and seals. Such divisions are recognized throughout the 

 animal kingdom and it is but few of them that arc not represented in the 

 sea, although the marine plants, except for the eel grass, all belong to one 

 phylum. There are no marine amphibians (frogs, newts etc.), but there are 

 marine reptiles (some turtles, sea-snakes, and the marine lizards of the 

 Galapagos Islands), and mammals (seals, dolphins, whales). All the inverte- 

 brate sub-phyla are represented, though there are extremely few insects, 

 millipedes or spiders compared with the Crustacea. Some phyla are totally 

 marine, notably the Echinodermata (starfishes and sea-urchins) and the 

 Chaetognatha (arrow-worms). Not all of these marine forms are planktonic 

 of course, but it is remarkable how many are planktonic at some stage in 

 their life history, and Chapter 6 will be devoted to the planktonic larvae of 

 bottom-dwelling species. It should be noted that the odd turtle that is carried 

 away from his normal habitat in Mexico and eventually stranded on the 

 coasts of Ireland or Scotland has drifted across in spite of his ability to swim, 

 and may thus be considered as aberrantly planktonic. These, however, are 

 the exceptions and of passing interest (Chapter ii). 



Many, very many, of the smaller marine creatures are planktonic all 

 their lives; the scientist calls them 'holoplanktonic' as against 'meroplank- 

 tonic', a term that includes organisms that are only planktonic during part 

 of their lives. The abundance of small creatures has given rise to the impres- 

 sion that the study of plankton is essentially a microscopist's calling; though 

 in general true, this is not completely so, and quite a lot can be done by the 

 naked eye or with a lens of only small magnification. Some of the largest 

 jellyfish can be a yard or more across. 



This and the next chapter will describe a selection of the commonest and 

 most characteristic types in the classified order, but dealing only with the 

 holoplanktonic species, and this will be very far indeed from an exhaustive 

 survey. 



D 43 



