Some seaweeds are eaten, as for example the so-called “Carrageen,” or Irish moss, which is used in both jelly and size, and is one of the Rhodosperm algæ. To preserve it for esculent purposes it is washed in fresh water and allowed to dry; it becomes then horny and stiff. If boiled it subsides into a thick jelly, which is considered nutritious, and is used by both invalids and epicures. Calico-printers use it for size. It is used, boiled in milk, to fatten calves.
A pretty little seaweed, Griffithsia selacea, has the property of staining paper a fine pinkish-scarlet hue when its membrane bursts. Contact with fresh water will usually cause the membrane to yield, and then the colouring-matter is exuded with a slight crackling noise.
The Chlorosperms, or green-seeded algæ, have the power of pouring out large quantities of oxygen under certain conditions, and are therefore very valuable in the aquarium. Among them are the sea-lettuce, before mentioned, the common sea grass, and a large number of smaller and more delicate forms.
“If,” says Mr. Wood, “the naturalist wishes to dry and preserve the algæ which he finds, he may generally do so without much difficulty, although some plants give much more trouble than others. It is necessary that they should be well washed in fresh water, in order to get rid of the salt, which, being deliquescent,[53] would attract the moisture on a damp day, or in a damp situation, and soon ruin the entire collection. When they are thoroughly washed the finest specimens should be separated from the rest and placed in a wide, shallow vessel, filled with clear fresh water. Portions of white card, cut to the requisite size, should then be slipped under the specimen, which can be readily arranged as they float over the immersed card. The fingers alone ought to answer every purpose, but a camel’s-hair brush and a needle will often be useful. When the specimen is properly arranged the card is lifted from the water, carrying upon it the piece of seaweed. There is little difficulty in getting the plants to adhere to the paper, as most of the algæ are furnished with a gelatinous substance which acts like glue and fixes them firmly down.” If not, the [pg 203]use of hot water will generally accomplish the desired end. Animal glue or gum-water cannot he recommended.
Every visitor to the sea-shore has observed rocks drilled with innumerable holes, almost as though by art. A few good blows with a stout hammer on the chisel-head serve to split off a great slice of the coarse red sandstone. The holes run through its substance, but they are all empty, or filled only with the black fœtid mud which the sea has deposited in their cavities. These are too superficial; they are all deserted; the stone lies too high above low-water mark; we must seek a lower level. Try here, where the lowest spring-tide only just leaves the rocks bare. See! now we have uncovered the operators. Here lie snugly ensconced within the tubular perforations, great mollusca, with ample ivory-like shells, which yet cannot half contain the whiter flesh of their ampler bodies, and the long stout yellow siphons that project from one extremity, reaching far up the hole towards the surface of the rock.
We lift one from its cavity, all helpless and unresisting, yet manifesting its indignation at the untimely disturbance by successive spasmodic contractions of those rough yellow siphons, each accompanied with a forcible jet d’eau, a polite squirt of sea-water into our faces; while at each contraction in length, the base swells out till the compressed valves of the sharp shell threaten to pierce through its substance.
Strange as it seems, these animals have bored these holes in the stone, and they are capable of boring in far harder rock than this, even in compact limestone. The actual mode in which this operation is performed long puzzled philosophers. Some maintained that the animal secreted an acid which had the power of dissolving not only various kinds of stone, but also wood, amber, wax, and other substances in which the excavations are occasionally made. But it is hard to imagine a solvent of substances so various, and to know how the animal’s own shells were preserved from its action, while, confessedly, no such acid had ever been detected by the most careful tests. Others maintain that the rough points which stud the shell enable it to serve as a rasp, which the animal, by rotating on its axis, uses to wear away the stone or other material; but it was difficult to understand how it was that the shell itself was not worn away in the abrasion.
Actual observation in the aquarium has, however, proved that the second hypothesis is the true one. M. Cailliaud in France, and Mr. Robertson in England, have demonstrated that the Pholas uses its shell as a rasp, wearing away the stone with the asperities with which the anterior parts of the valves are furnished. Between these gentlemen a somewhat hot contention was maintained for the honour of priority in this valuable discovery. M. Cailliaud himself used the valves of the dead shell, and imitating the natural conditions as well as he could, actually bored an imitative hole, by making them rotate. Mr. Robertson at Brighton exhibited to the public living Pholades in the act of boring in masses of chalk. He describes it as “a living combination of three instruments, viz., a hydraulic apparatus, a rasp, and a syringe.” But the first and last of these powers can be considered only as an accessory to the removing of the detritus out of the way when once the hole was bored, the rasp being the real power. If you examine these living shells you will see that the fore part, where the foot protrudes, is set with stony points arranged in transverse and longitudinal rows; the former being the result of elevated ridges radiating from the hinge, [pg 204]the latter that of the edges of successive growths of the shell. These points have the most accurate resemblance to those set on a steel rasp in a blacksmith’s shop. It is interesting to know that the shell is preserved from being itself permanently worn away by the fact that it is composed of arragonite, a substance much harder than those in which the Pholas burrows. Yet we see by comparing specimens one with another, that such a destructive action does in time take place, for some have the rasping points much more worn than others, many of the older ones being nearly smooth.
PHOLADES IN A BLOCK OF GNEISS.