ZOOPLANKTON I 



adhering to the skin but not yet exploded. Fortunately, it is easily recognized 

 by its large bluish or greenish float some 6 inches or so in length. This float, 

 which also acts as a sail, is set at an angle to the rest of the body, sometimes 

 to the left, sometimes to the right, with the result that they are differently 

 affected by the wind and a shoal consists only of those with the same set of 

 the sail. Because the dominant winds in an area are usually constant we are 

 not sure if the two types are produced in equal numbers and separated at an 

 early stage by the wind, one type tending to come ashore on the European 

 coast and the other going out to sea. It may be that the different angles are 

 associated with the two hemispheres, and those stranded in the southern 

 hemisphere usually have their sails set in the opposite direction from those 

 stranded in the northern hemisphere. 



A small harmless siphonophore, Pliysophom (Fig. 17; 2), is found in boreal 

 waters and is common in the northern North Sea, Norwegian Sea and 

 Iceland: ' waters, though it also penetrates much farther south, even into the 

 Mediterranean. A surface floating warm-water pseudo-siphonophore or 

 chondrophore is Velella, also known as the 'By-the-wind sailor' or 'sallee 

 man' (Fig. 17; 5). It is oval in shape, purple or blue and about i or 2 inches in 

 length. This species has a stiff sail set at an angle across the disc and this may 

 be set to the left or to the right as in Pliysalia with the result that almost all 

 those stranded on the west coast of the British Isles have their sails set to the 

 left. When they strand and so die their horny skeletons complete with sail 

 are blown up on the beaches. Common off the south-west coasts of Britain, 

 it is also found off the Hebrides, occasionally off Faroe and very rarely to the 

 south of Iceland. 



Another important and almost wholly planktonic phylum is the Cteno- 

 phora, which in older classifications was often placed in the Coelcnterata. A 

 number of species are known but only two are of sufficient abundance to 

 merit description here. 



The first is the sea gooseberry, Pleurobrachia piletis (Plate XVI), which 

 although quite widespread in its occurrence is especially abundant in 

 coastal waters, though strangely enough it is absent from the Icelandic shelf. 

 Its name is appropriate because it does look just like a transparent gooseberry 

 and the jelly is stiff enough for it to keep its shape when stranded on the 

 beach. These gooseberries swim by the beating, to and fro, of rows of 

 ciUa — whip-like hairs — arranged in eight series of combs (Fig. 17; 6) and 

 hence the name comb-jelly. They seem perfectly at ease any way up. They 

 have a pair of retractile tentacles which they can tuck away in pockets (the 

 name Pleurobrachia means 'folded arms') or extend to some ten times the 

 body length when capturing food. Unlike the true jellyfish they have no 

 stinging cells but capture their prey by entanglement in these rather sticky 



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