We mentioned the sponges a moment ago and many of them may be found around our coasts; not the household sponges we know so well, but much smaller, though equally interesting, colonies. Like the better known sponges, those which we find on our shores are perforated with a number of holes through which the water is driven by means of little whip-like structures which line the cavities. Professor Grant has graphically described his impressions at witnessing this water current for the first time. “I put a small branch of the Spongia Coalita,” he writes, “with some sea water in a watch glass, under the microscope, and, on reflecting the light of a candle up through the fluid, I soon perceived that there was some internal motion in the opaque particles floating through the water. On moving the watch glass, so as to bring one of the apertures on the side of the sponge fully into view, I beheld, for the first time, the splendid spectacle of this living fountain vomiting forth from a circular cavity an impetuous torrent of liquid matter, and hurling along, in rapid succession, opaque masses, which it strewed everywhere around. The beauty and novelty of such a scene in the Animal Kingdom long arrested my attention; but after twenty-five minutes of constant observation, I was obliged to withdraw my eye from fatigue, without having seen the torrent for one instant change its direction, or diminish, in the slightest degree, the rapidity of its course. I continued to watch the same orifice, at short intervals, for five hours, sometimes observing it for a quarter of an hour at a time, but still the stream rolled on with a constant and even velocity. About the end of this time, however, I observed the current to become perceptibly languid ... and in one hour more the current had entirely ceased.”
Frequently the sponge we examine may be found to be studded with many yellowish spots; closer examination will show that these spots are composed of very small jelly covered eggs. Later these eggs find their way into the cavities of the sponge and are forced therefrom in the currents of water. Each of the young sponges thus expelled is furnished with a covering of little whips, by means of which it swims about till it can find a suitable spot on which to anchor and complete its growth.
The Sea Anemone, which has already provided us with objects for our microscope, has many near relatives which we must make a point of examining, while we have the opportunity. Many of these creatures, or rather their colonies, for they do not live singly, are to the naked eye, strangely like seaweeds. A number of them are moss-like and may be found on wooden breakwaters and similar situations when the tide is low; they should be collected and examined and, to see them at their best they should be examined under water. It is hardly necessary to describe any one of these colonies in detail, for they are so numerous that the one we described might not come into the hands of our readers for a long time. In general characters they are all somewhat similar so we will confine ourselves to generalities. For the most part, the stems and branches of these colonies are of the thickness of thread. As we watch them under the microscope we shall see that they are studded with little cups and, presently, from each little cup there appears a little tuft of tentacles which is waved about in the water. Each member of the colony is similar to its neighbour and each one, again, is very like the fresh water Hydra with which we are familiar.
Of all the common objects of the sea shore one of the commonest everywhere is the sea-mat. Nine people out of ten or, we might safely say that everyone who had not learned its true nature, would guess it to be a seaweed. As we find it washed up on the beach it is almost the colour of sand, somewhat rough to the touch, whilst its whole surface is pitted with minute holes. The sea-mat, when dry as we usually find it, is a remnant of a colony of sea dwellers very similar to those we have just described. From each little hole, in a living specimen, which we can find without much difficulty, there appear the familiar tentacles; each hole is the home of a minute hydra-like animal.
Hooke, whom we mentioned in our [chapter on the History of the Microscope], though a careful observer, was quite misled by the sea-mat; he thought it was a seaweed, for he wrote: “I have not, among all plants and vegetables I have yet observed, seen any one comparable to this seaweed. It is a plant which grows upon the rocks under water and increases and spreads itself into a great tuft, which is not only handsomely branched into several leaves, but the whole surface is covered over with a most curious knot of carved work, which consists of a texture much resembling a honeycomb, for the whole surface on both sides is covered over with a multitude of very small holes, being no larger than so many holes made with a pin, and ranged in the neatest and most delicate order imaginable, they being placed in the manner of a quincunx, or very much like the rows of eyes of a fly, the rows or orders being very regular which way soever they are observed. These little holes, which to the eye look round, when magnified, appear very regularly shaped holes, representing almost the shape of a round-toed shoe, the hinder part of each being, as it were, turned in, or covered by the toe of the next below it. These holes seemed walled about with very thin and transparent substance, looking of a pale straw colour, from the edge of which, against the middle of each hole, were sprouted out four small, transparent, straw-coloured thorns, which seemed to protect and cover those cavities.”
As a well-known author has remarked: “This is really a wonderfully faithful description of the common sea-mat, and one cannot help picturing the surprise and delight of old author Hooke, could he have seen a portion of a living colony under a modern microscope.”
One of our finds may be the “Bird’s head.” It is a branched form, quite unlike the sea-mat but it is of even greater interest. Under the microscope, we shall see the many waving tentacles, but another feature is sure to attract our attention, a feature which is responsible for the popular name of the colony. On the outside of each cavity containing a member of the colony there is a structure which resembles nothing so much as a hawk-like bird’s head atop of a long neck. While the tentacles wave in the water, the bird’s head snaps vigorously, moved here and moved there. The birds’ heads, which might be mistaken for parasites stealing food from the waving tentacles, really perform the useful function of keeping them clean and warding off creatures which might do them harm.
By the courtesy of Messrs. F. Davidson & Co.