comes back to the heart by veins. In making this circuit it finds on its route the lungs filled with air by means of respiration.
In our caterpillar we also find blood and a species of heart, but it has neither arteries nor veins. The blood is diffused throughout the body and bathes the organs in all directions. However, it ought to respire. Here step in the openings of which I have spoken. They lead to a system of ramified canals, of which the last divisions penetrate everywhere, and carry everywhere the air—that fluid essential to the existence of all living beings. In our bodies the air and blood are brought together. In insects the air seeks the blood in all parts of the body.
I have sketched for you a caterpillar when it is full grown. But you well know that living beings are not born in this state. The general law is, small at birth, growth, and death. The caterpillar passes through all these phases.
| Fig. 3. | Fig. 4. |
 | |
| Egg and First Age, lasting five days. (An age is the interval between two moultings.) | Second Age, lasting six days. |
I pass around among you some samples of what we call seeds of the silk-worm. These so-called seeds are in reality eggs. The caterpillar comes out of the egg very small; its length, at birth is about one-twentieth of an inch. Look at these samples, and you will see how
| Fig. 5. | Fig. 6. |
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| Third Age, lasting six days. | Fourth Age, lasting six days. |
Fig. 7.
Fifth Age, lasting nine days. The mature worm near the end of its career, and at the time of its greatest voracity.
great is the difference of size between the worm at birth and the fullgrown specimens I have shown you. This difference is much greater than in man. A man weighs about forty times as much as the new-
