CRUSTACEANS 3 5 



In the sea the Copepods both as regards species and 

 individuals are indeed legion. One abundant form, 

 Calanus, deserves to be as well known as the crab or 

 lobster since it is infinitely more abundant, and without it 

 the herring harvest would fail to reach maturity. This 

 minute crustacean may so abound at times as to cover some 

 square miles of the sea surface with a reddish scum and 

 a single tow of a few minutes' duration has choked a muslin 

 net with two and a half million specimens — sufficient to 

 fill ten pint tumblers solid. 



Some Copepods are of exquisite form and colour, whilst 

 many display many fantastic spines and fancy appendages, 

 which serve to maintain their balance. 



The group known as Cirripedia includes a large number 

 of strangely divergent forms spoken of collectively as 

 Barnacles. The majority commence life as active free- 

 swimming creatures, but after casting their shells several 

 times they attach themselves by the head to some solid 

 object — rock, stone, crag, or ship's keel, and there undergo 

 a startling transformation. The shell or carapace which 

 encloses the larval body changes into a tower-like structure 

 made of several neatly welded plates, the upper or open 

 end being closed by two pairs of folding doors, which 

 can be pushed aside to admit the free passage of the legs. 

 The latter organs no longer of service in rowing the animal 

 through the water serve merely, as Professor Thomas 

 Huxley aptly put it — to kick the food into the stomach. 



By far the best known example is the little Acorn 

 Barnacle (Balamis balanoides), which covers every rock and 

 harbour pile, its sharp-edged shells being justiy disliked 



