CHAPTER 5 



The (iiiiiiials — the zoophiiihtoii II 



Continuing the brief description of some of the commonest and most 

 interesting species of plankton animals we come to perhaps the most 

 important phylum of all — the Arthropoda, creatures with jointed legs, and 

 the most important marine sub-phylum, the Crustacea. Included in the 

 Crustacea are the crabs, lobsters and shrimps which crawl about the sea floor, 

 the attached barnacles, freshwater shrimps and creatures like the garden 

 woodlouse. There are, also, hosts of truly planktonic crustaceans, in fresh 

 water as well as in the sea, and they form one of the vital links between the 

 plants of the phytoplankton and the fish, or those creatures that so often form 

 the food of the fish . 



Belonging to one of the simplest types of the Crustacea, the sub-order 

 Cladocera, are Podoii and Ei'adiie which are frequently abundant and are 

 easily recognized from Fig. 19; i, 2, and Plate XVII. Small crustaceans with 

 a bivalve shell reminiscent of molluscs, in the class Ostracoda are very common 

 living in or near the bottom in both fresh and salt water, but some are 

 planktonic, particularly in oceanic waters (Fig. 19; 3). 



The most prolific and most important class is, however, the Copepoda — 

 oar-footed — and most of these are planktonic. Some are parasitic and are 

 quite unlike the free-living forms and are not the concern of this book, others 

 live closely attached to the surface of fronds of weed but they are usually 

 easily enough identified as copepods. The free-living ones have a well- 

 defined head-plus-body called a 'cephalosome' with antennae, mouth parts 

 and swimming feet, and a tail region or abdomen called a 'urosome' which 

 has no appendages except a bifurcated tip provided with hairs (setae). Both 

 cephalosome and urosome are jointed, but one special joint is used in classifica- 

 tion. In most of the truly planktonic marine species it is between the 

 cephalosome and the abdomen so that there are no appendages behind this 

 joint (Fig. 20) ; in most of the rock-pool and freshwater species it is in front of 

 the last segment of the cephalosome so that the 'tail' has an extra joint which 

 carries a pair of reduced swimming feet (Fig. 18; i — 4). Both kinds are found 

 in the marine plankton, in rock pools and in fresh water. Planktonic copepods 

 vary in size from very tiny, about h millimetre or 5^ inch, to quite large, 

 about 12 millimetres or | inch, but the largest you are likely to find without 

 an ocean-going vessel equipped to sample deep water will be about the size of 

 a grain of rice. Some deep-water forms are quite big, for example, Paretichaeta 



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