There are some species peculiar to hot climates, where they are generally larger, and more luminous; and some to cold. But what is wanting in size is amply made up in number, in the Arctic regions. A certain naturalist calculated by a fair average, how many Acalephæ there were in two square miles of sea, and the result of his calculation would fill up a whole line of this work with figures; you may judge something of it, by his saying that to count the number would have taken 80,000 persons all the time that has elapsed since the creation of the world, counting as fast as they could! It is on these creatures, most of them so small that they cannot be seen without a microscope, that the vast bodies of the whales are supported, caught in the wonderful shrimp-trap, which I described to you in a former page! Two or three kinds are found on your own shores, and I should not wonder if some of you may not before now have found them lying on the sea-shore, which the tide has left dry, and taken them up in your hands. If so, I am sure you will not forget it, nor be in any difficulty to know why they are called _sea nettles_. They have a stinging power, which will make the hand smart that touches them. This is owing to a caustic fluid with which a part of their bodies is constantly covered. It is this caustic fluid which is luminous. It oozes through the skin all round the muscular ring by which they move, and at the large filaments. The whole body of the creature looks bright, but it is only from the light transmitted by these parts. You may get a very fair notion of the appearance of one of their bodies, by rubbing together two partly transparent pebbles in the dark; the light is of nearly the same colour, and though it is only produced just at the points of contact of the pebbles, it illuminates their whole substance. As they move along they are much brighter when they contract their bodies, than when they expand them; this is because in contracting they press out the luminous fluid. I will tell you of some experiments and observations, which have been made on them. The body of an Acalepha was squeezed over a glass of warm fresh water, and the fluid that dropped out communicated its luminous property to the water. The same was then done with a glass of warm salt water, but the effect was not nearly so great. One was squeezed over a vessel containing nearly a quart of milk, which it made so resplendent that one could see to read by it. The milk retained its brightness for several hours, and when it faded, it could be restored by stirring: even three days afterwards it was made bright by being warmed. In this manner the Acalephæ communicate a slight degree of light to the sea-water, in which they swim; but, if they are put into fresh water, the light spreads much further.