NATURE ADRIFT 



The next order is the Amphipoda, probably more famiharly represented 

 bv the freshwater 'shrimps' and the sand hoppers often so abundant amongst 

 rotting seaweed stranded on the seashore. They will be commonly found, 

 too, in the rock pool fauna but are less typical of the plankton of the open 

 sea. There are some grotesque planktonic amphipods from deep water but 

 only a few species are to be found in the surface or inshore waters. Strangely 

 enough, when they do occur they can be very abundant indeed. Themisto, 

 for example (Fig. 19; 5), is another truly surface form, and like the copepod 

 Amviialoccra frequently causes a 'rain' on the surface of calm water by 

 jumping up and down, but is rarely taken in large numbers in sub-surface 

 nets. Its outer surface has the property — surprising for a marine creature — 

 of being unwettable so that if any part of it comes above the water the 

 effect of the surface tension brings the whole of the rest out to float, quite 

 dry. Unless it is pushed or wriggles completely below the surface, out it 

 comes again, a most disconcerting habit when one is trying to look at it 

 through a microscope. Another amphipod, closely allied to Themisto, is 

 called Hypcria (Fig. 19; 6) and is found most commonly in the plankton 

 close to the larger jellyfish as it is an ectoparasite on them, feeding on the 

 jelly itself 



As in other groups, there are many exotic kinds ot deep-water amphi- 

 pods, some brightly coloured, some very swollen and transparent such as 

 Mimouectcs (Fig. 22; 3). 



The Isopoda are typically represented by the familiar garden wood 

 louse, but there are also freshwater and marine species, including one, Ligia 

 occaiiicci, that runs round the rocks of the seashore at or above high-water 

 mark just as the wood louse does on land. Structurally they rather resemble 

 the amphipods but are usually flattened dorso-ventrally (hke a wood louse) 

 instead of from side to side as in the amphipods, and their legs are of almost 

 equal length — hence the name. Some quite big ones, Idotliea is about |-i inch 

 long, live in the rock pools (Fig. 19; 8). Planktonic Idotlica arc almost always 

 associated with floating weeds and can be carried hundreds of miles by the 

 currents, crawling about the fronds, occasionally swimming a little, and 

 several generations can live in their little drifting patch, perhaps only a 

 foot across. Etirydicc (Fig. 19; 7), is a truly planktonic isopod; there are 

 also some planktonic deep-sea isopods and a whole book could be written 

 about those peculiar isopods which are parasites of other crustaceans. 



Further up the crustacean series of orders there are the Mysidacea, the 

 fairy shrimps (Fig. 22; 2) rather shrimp-like in shape, usually extremely 

 transparent except for their eyes which are usually very dark but in some 

 species are bright red, orange or opaque white. They have a short carapace 

 which encloses the thoracic region of the body but is only fixed at the front, 



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