THE INVISIBLE FOOD OF FISH 245 



seems to kill these entomostraca. They swarm in the 

 distilled brine of the salterns on the Solent. Their 

 eggs are proof against frost, and survive being baked 

 by the sun. They even come to life without being 

 fertilised. Yet they undergo infinite changes of form, 

 and their cast shells are piled like billows of dust 

 on parts of the Cornwall coast. Detached and self- 

 supporting, they wander over the whole ocean, swim- 

 ming mainly upon the surface. At times they descend 

 to the deeps, and this, it is surmised, causes the 

 temporary disappearance of shoal fish, which necessarily 

 follow them. Their countless numbers are also re- 

 cruited by the microscopic larvae of fixed shells. The 

 barnacle, for instance, begins life in this form, taking 

 its place in the ingredients of the { sea-soup ' as a 

 one-eyed swimming crustacean, then growing a pair of 

 eyes, and finally settling down as a fixture in proper 

 barnacle style. 



In rivers they are almost the sole food of all young 

 fish, and probably the main resource of the older fish 

 when other supplies fail. In the first days of spring, 

 the creatures in every stage, eggs, larvae and perfect, 

 though microscopic entomostraca, swarm in the water, 

 on the mud, and on the foliage of the water-plants. 

 At such times even trout feed mainly on them. In 

 the Hertfordshire streams the trout are then said to 

 be ' tailing.' They push their heads down into 



