NATURE ADRIFT 



a single blob of frog spawn. It can, indeed, be parasitized by some of the 

 dinoflagellates (p. 41). 



Also important are those protozoa attached to larger planktonic animals 

 (Fig. 12; 2a and h: Plate X). As these feed on bacteria and detritus they are 

 particularly abundant in estuaries which are polluted by untreated sewage and 

 here they have a particularly useful function in the destruction of sewage 

 bacteria. Their attachment to relatively active creatures ensures a continual 

 change of water which thus provides an ample supply of their food, and in 

 turn they are fed on by other animals, often by those which have carried them 

 about. In a polluted estuary, e.g. the Mersey, Humber, Thames, and even 

 places like the Oslo fjord, these abundant protozoa form the food of small 

 Crustacea, which are in turn fed on by the larger creatures. The local plankton 

 is thus not so dependent upon the growth of plant life as it would be in a 

 non-polluted area, and a good rich growth can exist even in winter con- 

 ditions when the light is poor. 



Coclciitcrata (Jcllytish, sea anemones, corals etc.) 



Coelenterates are all aquatic, and with only a few exceptions are marine. 

 Although a large proportion of coelenterates are attached at some part of 

 their life history, some of the most characteristic are always free-swimming. 

 The planktonic stage of jellyfish is so typical of the plankton that it should be 

 considered here rather than in Chapter 6 which deals with the meroplank- 

 tonic forms. The feathery hydroids common in pools and on shore seaweeds 

 produce small medusae (jellytish) which arc then part of the plankton. 

 These small jellies, some of which are illustrated in Figs. 13 and 14, and in 

 Plate XI, are vastly more abundant than the large conspicuous ones that annoy 

 the bathers and fishermen so much. All are carnivorous, feeding on other 

 planktonic creatures including small fish (Plate XIV). These they capture by 

 paralysing them with their stings which are usually prolific, especially on their 

 tentacles and round the mouth region, though they are usually absent from 

 the upper side of the 'bell'. Each stinging cell is called a 'nematocyst' and there 

 are quite a variety of types. Basically each is a sac of poison with an invagin- 

 ated barb — like the inside-out finger of a glove, though much elongated 

 (Fig. 15). When touched the sac is compressed, out shoots the barb, penetra- 

 ting the skin of the unfortunate creature, and acting as a tube through which 

 the poison is injected. The paralysed prey is then easily eaten. Jellyfish have 

 small 'statocysts', little balancing organs with which they can not only sense 

 which way up they are but can feel vibrations in the water caused by the 

 turbulence of other moving creatures. So accurately can they sense these 

 vibrations that, although quite unable to sec. they can stretch out their mouths 



48 



