In answer to a letter in which I asked the writer for some further information concerning the Velella, sending also an outline sketch of the animal, which I asked the writer to fill in with the proper colours, I received the following reply:—
“I will do my best to answer your questions, and to give you what information I can concerning the creatures.
“When seen at Tenby, they were all floating on the surface of the sea, the tentacles only being submerged. My specimens floated for a very short time after capture, death following so quickly that I was obliged to set to work at once with camel’s-hair brush and penknife to take away the gelatinous part. Indeed, decomposition took place so rapidly, that Velellas and myself were simultaneously threatened with extermination.
“Both raft and sail were equally enveloped in a soft, gelatinous covering, certainly not more than the sixteenth of an inch in thickness, except under the centre of the raft, where it became slightly thicker. The covering of the sail was exceedingly thin, and like a transparent and almost invisible soft skin. The sail is very firmly attached to the raft, as they did not separate when decomposition began.
“The tentacles were entirely composed of the same soft, jelly-like substance as that of the envelope, and every part was iridescent in a sort of vapoury transparent cloud of many-tinted colours, blue and pale crimson predominating. I have filled up to the best of my memory the little sketch, and only wish you could have seen the Velellas as I did, in their full life and beauty.”
Two of the specimens here mentioned are in my collection, and beautiful little things they are. The two plates are not thicker than ordinary silver paper, but are wonderfully strong, tough, and elastic. The oval horizontal plate, or raft, if it may be so called, is strengthened by being corrugated in concentric lines, and having a multitude of very fine ribs radiating from the centre to the circumference. It is slightly thickened on the edges, evidently for the attachment of the tentacles.
The perpendicular plate, or sail, does not occupy the larger diameter of the raft, but stretches across it diagonally from edge to edge, rising highest in the centre and diminishing towards the edges, so that it presents an outline singularly like that of a lateen sail. It is rather curious that the magnifying glass gives but little, if any, assistance to the observer, the naked eye answering every purpose. Even the microscope is useless, detecting no peculiarity of structure. I tried it with the polariscope, scarcely expecting, but rather hoping, to find that it was sensitive to polarised light. But no such result took place, the Velella being quite unaffected by it.
The corresponding illustration is a sketch of a raft to which a sail is attached. Such rafts as this are in use in many parts of the world, the sail saving manual labour, and the large steering oar answering the double purpose of keel and rudder. In the Velella, the tentacles, though they may not act in the latter capacity, certainly do act in that of the former, and serve to prevent the little creature from being capsized in a gale of wind.
There is no doubt that the first idea of locomotion in the water, independently of swimming, was the raft; nor is it difficult to trace the gradual development of the raft into a Boat. The development of the Kruman’s canoe into the Great Eastern, or a modern ironclad vessel, is simply a matter of time.
It is tolerably evident that the first raft was nothing more than a tree-trunk. Finding that the single trunk was apt to turn over with the weight of the occupant, the next move was evidently to lash two trunks side by side.
Next would come the great advance of putting the trunks at some distance apart, and connecting them with cross-bars. This plan would obviate even the chance of the upsetting of the raft, and it still survives in that curious mixture of the raft and canoe, the outrigger boat of the Polynesians, which no gale of wind can upset. It may be torn to pieces by the storm, but nothing can capsize it as long as it holds together.
Laying a number of smaller logs or branches upon the bars which connect the larger logs is an evident mode of forming a continuous platform, and thus the raft is completed. It would not be long before the superior buoyancy of a hollow over a solid log would be discovered, and so, when the savage could not find a log ready hollowed to his hand, he would hollow one for himself, mostly using fire in lieu of tools. The progress from a hollowed log, or “dug-out,” as it is popularly called, to the bark canoe, and then the built boat, naturally followed, the boats increasing in size until they were developed into ships.
Such, then, is a slight sketch of the gradual construction of the Boat, based, though perhaps ignorantly, on the theory of displacement. Now, let us ask ourselves whether, in creation, there are any natural boats which existed before man came upon the earth, and from which he might have taken the idea if he had been able to reason on the subject. The Paper Nautilus is, of course, the first example that comes before the mind; but although, as we have seen, the delicate shell of the nautilus is not used as a boat, and its sailing and rowing powers are alike fabulous, there is, as is the case with most fables, a substratum of truth, and there are aquatic molluscs which form themselves into boats, although they do not propel themselves with sails or oars.
Many species of molluscs possess this art, but we will select one as an example of them all, because it is very plentiful in our own country, and may be found in almost any number. It is the common Water-snail (Limnæa stagnalis), which abounds in our streams where the current is not very strong. Even in tolerably swift streams the Limnæa may be found plentifully in any bay or sudden curve where a reverse current is generated, and therefore the force of the stream is partially neutralised. These molluscs absolutely swarm in the Cherwell, and in the multitudinous ditches which drain the flat country about Oxford into that river as well as the Isis.
Belonging to the Gasteropods, the Water-snail can crawl over the stones or aquatic vegetation, just as the common garden snail or slug does on land. But it has another mode of progression, which it very often employs in warm weather. It ascends to the surface of the water, reverses its position so that the shell is downward, spreads out the foot as widely as possible, and then contracts it in the centre, so as to form it into a shallow boat.
The carrying capacity of this boat is necessarily small, but as the shell and nearly the whole of the animal are submerged, and therefore mostly sustained by the water, a very small amount of flotative power is sufficient for the purpose. Sometimes, on a fine day, whole fleets of these natural boats may be seen floating down the stream, thus obtaining a change of locality without any personal exertion.
In perfectly still water, where no current can waft the Limnæa on its easy voyage, it still is able to convey itself from one place to another. By means of extending and contracting the foot, it actually contrives to crawl along the surface of the water almost as readily as if it were upon the under side of some solid body, and, although its progress is slow, it is very steady. Another very common British water-snail, the Pouch-shell (Physa fontinalis), has almost exactly the same habits. Reference will be made to the Pouch-shell on another page.
The capacity for converting the body into a boat is not confined to the molluscs, but is shared by many other animals. Take, for example, the well-known marine animals, called popularly Sea-anemones. As they appear when planted on the rocks, they look as incapable of motion as the flowers whose names they bear. Yet, by means of the flattened base, which they use just as a snail uses its feet, they can manage to glide along the rocks in any direction, though very slowly.
The base is capable of extension and contraction, and by elongating one side of it, fixing the elongated portion, and then raising the remainder of the base towards it, the animal makes practically a series of very slow steps. This mode of progression may often be seen in operation on the glass front of an aquarium.
The same property of expansion and contraction enables the Sea-anemones to convert their bodies into boats, and float on the surface of the water. When one of