NATURE ADRIFT 



There are a number of more oceanic species of pteropods which range 

 from arctic to tropical seas, and some of these, hke the arrow-worms are 

 carried by currents to more inshore or mixed waters. They do not all have 

 twisted shells (Fig. i8 ; 6) and some have no shell at all. typical of the unshelled 

 pteropods is Cliotic (Fig. i8 ; 9) which, like Spiratella, is found quite commonly 

 in inshore areas if there is a good admixture of oceanic water. Only small 

 ones, about ^ to J inch long are usually found in the North Sea, but farther 

 north and in the open ocean they are much bigger and reach almost an inch 

 off Iceland and in Greenland waters. Pteropods feed by filtering the phyto- 

 plankton and other small particles out of the water. 



Another planktonic group of molluscs is the Heteropoda, very trans- 

 parent rather slug-shaped animals which swim upside down with the mere 

 remnant of a shell hanging below them and acting as a keel. They are all 

 warm-water creatures but, carried by the currents, do occasionally reach the 

 temperate zones such as west of the British Isles. They are carnivorous, 

 feeding on fish and other quite rapidly moving creatures. 



Although it is not always appreciated, the squids and octopuses — the 

 Cephalopoda — arc also molluscs and they arc most interesting animals. Some 

 of course are bottom dwellers, others like the big squids are powerful 

 swimmers and not therefore planktonic, but there are a host of small ones 

 that are. They belong to both types — the octopuses which have eight arms 

 and the squids which have ten, eight short and two long (Fig. 18 ; 10 and 11). 

 Both kinds swim moderately slowly forwards by the use of their fin-like 

 extensions of skin or can shoot rapidly backwards by jet propulsion. To do 

 this they fill the cavity between the body organs and the outer skin or 

 mantle with water, and then powerfully eject it through a nozzle at the 

 base of the neck. As this nozzle is movable they can control the direction of 

 movement. Like the larger and better known cephalopods they produce 

 ink — sepia — and when escaping from an enemy they pour a cloud of it into 

 the water, like a smoke screen, and then dart away in what must to the 

 predator be an unpredictable and unseen direction. This can be further 

 complicated by a simultaneous change of colour in the animal, while the 

 cloud left behind is often shaped like the animal. There is such a size 

 range that it is impossible to make a dividing line between those that are 

 planktonic and those that can swim well enough to be called 'nekton'. 

 Certainly many of those big enough normally to be considered nekton are 

 occasionally drifted in shoals and stranded on the bcaclics, and included in 

 this list could be the common cuttlefish Sepia .ofjiciiinlis and the common 

 squid Loligo forbesii. The quick dart of some oceanic species is sufficient to 

 make them shoot into the air and glide along like flying fish, often for 50 

 feet or more and up to 12 or even 20 feet above the sea surface. Carnivores 



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