NATURE ADRIFT 



of these arrow-worms and other plankton animals as indicators of water 

 movements will be dealt with in Chapter ii. 



Molluscs 



Familiar to everyone arc the mussels, cockles and winkles of the sea 

 shores. These and many more marine molluscs live on or in the sea floor 

 or attached to rocks, and quite a large number of them have young plank- 

 tonic larvae which will be mentioned in Chapter 6. However, there are 

 some that are truly planktonic, even as adults, and thus belong to this 

 chapter. The most abundant, and the most important in the food chains of 

 the sea, is a small sea-snail now called Spimtclla rctrovcrsa but which has 

 usually been called Liiuaciiia rctrovcrsa. 



This changing of scientific names deserves a paragraph of explanation. 

 The use of the binomial system — a generic name and a specific name — 

 was first started by Linnaeus and is the basis of all biological nomenclature. 

 It is designed to prevent the misunderstanding that can arise with the use 

 of popular names that so often mean different things in different localities 

 and dialects even in the same language. Any system that is to be internation- 

 ally rigid must be governed by equally rigid rules and one of these is that 

 the earliest name given in a proper description is the one to be used unless 

 this name has previously been used for something else, hi this example the 

 name 'les Limacines' was given to it by Cuvier in [817 as a popular term, 

 and Blainville also in 18 17 gave it the name Spiratclla in a proper description. 

 However, in 18 19 Lamarck properly described it under the name Limacina 

 as distinct from the popular term 'les Limacines' as he presumably was 

 unaware of Blainville's work. Most biologists followed Lamarck as his 

 work was very widely known, but those in authority rightly decided that 

 under the rules Blainville's name must stand. 



Spiratclla (Plate XVI and Fig. 18; 7) is only about i/ioinch and grey 

 to black in colour. It often occurs in dense shoals where oceanic and coastal 

 waters mix and herring will then feed on it to the exclusion of their normal 

 food. As Spiratclla gives out a dark sepia-coloured stain the herring insides 

 become stained; they have what it known to the herring trade as 'black gut'. 

 This is accompanied by a rather foetid smell and the fish do not preserve 

 well. Plankton containing a lot of Spiratclla will also become stained in the 

 jar and the inside label will turn a dark plum colour. In life they swim with a 

 dancing movement using two extensions of their bodies as wings or paddles, 

 and together with the other planktonic molluscs in the order Pterc^poda, they 

 have thus earned the name of sea butterflies. It is interesting to note that 

 while most of the winkles and snails have shells that are right-hand spirals 

 Spiratclla is left handed. 



60 



