ZOOPLANKTON I 



opening of the thimble — and it not only feeds mostly on other ctenophores 

 but is a voracious cannibal. 



Worms 



A wide range of really very different kinds of animals are loosely grouped 

 together as 'worms'. Although the term is convenient enough for a popular 

 book like this, they are so unrelated that such a grouping is a zoological 

 miscellany. Only a few of them are truly planktonic although there are 

 vast numbers of marine worms, free living, tube dwelling, internal and 

 external parasites. The true worms in the plankton are mostly warm-water 

 oceanic species that occasionally get drifted into the more temperate areas 

 as exotics, for example Lagisca litibrccliti (Plate XV). Only one need be 

 further mentioned here as it is frequently found in the mixed oceanic and 

 coastal waters of the North Atlantic. This is Tomoptcris lielgolaiidica, illus- 

 trated in Fig. 17; 9. It has a series of bi-lobed paddles on each side with 

 which it can swim energetically but nevertheless it is difficult to see in life 

 as it is so transparent. 



The arrow-worms or glass-worms (Fig. 18; 5) need a more detailed 

 description. They form the phylum 'Chaetognatha' and apart from one 

 species that lives amongst the grains of sand and broken shell on the bottom 

 they are all planktonic. They are extremely transparent in life, swimming 

 with a shimmering movement or by quick darts and they are very voracious 

 carnivores. Each is provided with a series of powerful hooks, and one or 

 more (usually two) double sets of small teeth, and a distendible pharynx 

 through which it can push creatures almost as big as itself. 



Over forty species are known and of these there are about a dozen in 

 the North Atlantic but only two are common in inshore waters, Sagitta 

 setosa, and Sagitta elegaiis (Plate XXXVI). Sagitta setosa is found in the 

 southern North Sea, English Channel, Clyde and Irish Sea, and in other 

 temperate places of relatively low salinity such as the Black Sea, parts of 

 the Adriatic, estuaries like the Gironde; there is a record also from near 

 Labrador. It is not found in the very low salinities of the Baltic nor in the 

 arctic or boreal regions, nor in the tropics. The other common species, 

 Sagitta ekgaiis, is found in the rather higher salinities where oceanic and 

 coastal waters mix such as the northern North Sea, western English Chaimel, 

 Irish Sea, and around Faroe, Iceland and near the North American coast. 

 The other ten species of the North Adantic are all oceanic, some warm- 

 and some cold-water inhabitants, some belonging to the surface layers and 

 others to the deep or very deep waters. Thus each has a habitat of its own 

 but can be carried into other environments by the currents; the value 



E 59 



