NATURE ADRIFT 



numbers, 9 to a cubic centimetre or 140 to the cubic inch of water, 

 and because of its position on the shore it can be flooded with rain, 

 dried out to a salt cake, frozen, or warmed to ioo°F by the sun. Few 

 hving creatures are so resistant, and because of this they are often used for 

 physiological experiments — the guinea-pigs of the sea! 



The sexes are separate in copepods and after mating the females produce 

 their eggs. These arc sometimes just liberated freely into the water or carried 

 about by the female in egg-sacs. Most of the rock-pool females carry their 

 eggs and in Ti^riopus, for example, the unripe eggs are dark green but turn 

 orange, just before they are ready to hatch One mating can last for several 

 batches of eggs in this species and, if this is general, it may help to account for 

 the fact that, although mature males of Calaiiiis are scarce compared with 

 females, there never seems to be a scarcity of fertilized females. This lack of 

 males is probably due to quicker development and a shorter life and not to a 

 difference between the numbers hatched. 



The young larvae that hatch out of the eggs are called nauplii (Fig. 21), and 

 have :it this stage three pairs of 'legs'. Though used for swimming at this stage 

 they are the beginnings of what will later be the antennae and mouth-parts. 

 This first nauplius soon casts its skin — a feature of all the Arthropoda — and the 

 second nauplius is slightly bigger. This procedure continues until the fifth or 

 sixth stage according to the kind of copepod, but the number never varies — 

 e.g. always five in Tiqriopiis, always six in Calaims, extra appendages being 

 formed stage by stage. The last nauplius then moults into something much 

 more like the parent, and is called a copcpodite. Again after successive moults, 

 with the appendages better formed at each stage, the adult is reached as the 

 mature sixth copepodite, and so the story is repeated. The number of genera- 

 tions per year varies according to species and locality. Many rock-pool 

 copepods have several generations in a season, the colder water planktonic 

 forms have usually only one a year. When there is only one, the eggs are laid 

 in the spring when there is phytoplankton present for the young nauplius to 

 feed on; they continue to grow and moult during the season spending the 

 winter as the fifth copepodite and finally maturing in the early spring in time 

 to mate and reproduce. 



Whilst it is generally true to say that most parasitic copepods live attached 

 to their hosts there are a few which can be caught in the plankton. These 

 are 'sea lice', external rather than internal parasites of fish, which are either 

 washed off or voluntarily leave their hosts, perhaps to find a new one. The 

 commonest of these is Cnlii^iis (Fig. 19; 9). 



The next class of Crustacea to consider here is the barnacles, the Cirripedia. 

 Barnacles are of two main types the 'acorn' type attached to rocks, which have 

 a planktonic life history and so are due to be dealt with in the next chapter, 



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