LIMITATION OF FAMILIES 129 



the course of the season, and watches over her young until they 

 have matured. The great water-beetles spin cocoons of silk in 

 which the eggs are deposited, and suspend them to a leaf or water- 

 weed. The sexton-beetles bury the carcasses of small animals and 

 lay their eggs in them. Butterflies, moths and most of the true 

 flies and bugs present every gradation from the almost random 

 deposition of a very large number of eggs to the careful selection 

 of a food-plant or food-material on which the eggs are laid, the 

 number being then smaller. Occasionally the eggs are deposited in 

 burrows that are excavated in the tissues of plants or in wounds 

 made in the bodies of animals. 



Many of the marine molluscs lay enormous numbers of eggs and 

 make no provision for the young. The common edible oyster 

 begins to breed when it is three years old, and the spawning season 

 lasts from AprU to August, beginning rather later in cold years. 

 The eggs hatch out inside the gill-chamber of the parent and emerge 

 as little free-swimming larvae, and it has been calculated that 

 from three hundred thousand to six millions may be discharged 

 by one oyster in a single season. A very large proportion of the 

 embryos perish, for they die unless they succeed in finding 

 suitable ground to fix themselves within a day or two. Those 

 that adhere to some solid object, such as a piece of stone, 

 lose their cilia and begin to grow rapidly, being small oysters 

 about an inch across at the end of the first year and thereafter 

 increasing at the rate of about an inch a year. In the common 

 fresh-water mussel, although the number of eggs is still very 

 large, being from fourteen thousand to a million, development 

 has proceeded further before the embryos are discharged from the 

 gill-chamber of the mother. The ciliated free-swimming stage is 

 passed through before hatching, and for a few hours the tiny embryo 

 swims about inside its own eggshell, thus recalling the free-swimming 

 state of its remote marine ancestors. After hatching, the embryo, 

 still within the giU-chamber of its mother, grows into a peculiar 

 larva known as the glochidium, with a shell consisting of two 

 valves hinged together, with strong teeth on the free margin of 

 each shell and with a long, sticky thread protruding from between 

 the shells. These larvae are then ejected by the mother into the 

 water, and they fall in masses to the bottom, the long, sticky threads 

 forming a tangled mass, like the web of a spider. Most of them 

 die, but if any small fish, attracted by the gelatinous mass of larva, 

 come near, then the glochidia become excited, flap their shells 



C.A. I 



