130 CHILDHOOD OF ANIMALS 



actively, and so straighten out the byssus thread. If one of these 

 threads touches a fish, it adheres, and the tangled mass, consisting 

 perhaps of many hundred larvae, is dragged behind the fish. As the 

 fish wriggles about, some of the glochidia are sure to be brought in 

 contact with it, and any that do so at once seize hold firmly with 

 the toothed edges of their shells, snapping them tightly together. 

 Those that have laid hold of a hard spine soon die and drop off. 

 Those that have lighted upon one of the gill filaments or the fleshy 

 part of fin or tail cause a slight inflammation in the tissues of the 

 fish and by the growth of these become enclosed in a cyst. Within 

 this they live on the juices of their host, are carried about by it, 

 and go through the rest of their development until they are perfect 

 little mussels. In the meantime, just as a thorn that has not been 

 extracted is gradually sloughed out of the human skin, so the 

 cysts containing the mussels are set free from the fish, and the 

 mussels drop into the mud and begin the normal adult life of their 

 kind. The fish that are chosen as the walking nurseries and feeding- 

 ground of the young mussels are usually sticklebacks, but minnows 

 and loach serve equally well. 



In very many molluscs, such as the common limpet, the eggs are 

 discharged directly into sea-water without any kind of protection 

 or provision for the young, and only a very small proportion of the 

 enormous numbers produced succeed in reaching adult life. In 

 others the numbers are much reduced and the eggs are enclosed in 

 special cocoons of various shapes, whilst a nutritive juice is placed 

 in the cocoon with them. In the common whelk these cocoons are 

 globular, and over a hundred of them, each containing about a dozen 

 eggs, are stuck together in a rounded mass. The embryos which 

 happen to develop first, however, eat their slower fellows, so that 

 only a very few actually leave the cocoons. Various devices, such 

 as the formation of floating rafts of mucus, often curiously shaped, 

 are adopted by other marine molluscs. Many of the fresh-water 

 molluscs fasten their eggs in strings to water-weeds, whilst a few of 

 those in the sea carry them attached to their own shells. Precisely 

 as such provision for the protection of the young becomes more 

 efficient, the number produced decreases. In Paludina, a common 

 English fresh- water snail, the eggs are developed inside the body 

 of the mother, and the young, comparatively few in number, are 

 not born until they are plainly young snails. 



In the air-breathing land snails, which must be regarded as the 

 most highly developed members of the group, the eggs are rather 



