Homes Without Hands: Being a Description of the Habitations of Animals, Classed According to Their Principle of Construction
“Such operations are protracted during several hours, though so gradually as to be apparently of little effect; nevertheless, on resuming inspection next morning, a surprising elongation of the tube will be discovered; or, perhaps, instead of a simple accession to its walls, the orifice will be surrounded by forking threads of sandy particles agglutinated together.
“The architect has now retired to repose; but, as evening comes, its activity is renewed, and again at sunrise a farther prolongation has augmented the extent of its dwelling.
“At first sight the numerous tentacula seem only so many long, cylindrical, fleshy threads of infinite flexibility.
“On examining them, however, more attentively, we see that in exercising their special function, the surface which is applied to the foreign objects becomes flattened into twice or thrice its ordinary diameter; and while conveying the sandy materials to the tube, these are seized and retained in a deep groove, which almost resembles a slit; in fact, the tentaculum becomes a flat, narrow ribbon, folding longitudinally in different places to hold the particles securely.
“Although these organs, when contracted, are collected into a brush scarcely double the thickness of the animal’s body, so enormous is their extensibility that they can be stretched out to the length of four inches, or half the length of the body, thus sweeping the area of a circle eight inches in diameter.
“A thin internal coating, resembling silk, lines the whole tube, and at the same time serves as a real cement to unite and strengthen its innumerable parts. This silk-like material is derived from a glutinous slime, which exudes from the surface of the body of the Terebella.
“Notwithstanding the unrivaled expertness and expedition with which this Annelidan advances its work, it has never been observed to resume possession of its tube when once forsaken. To obtain the shelter of a new dwelling in place of the old, its labors are invariably recommenced from the foundation.”
“In Terebella nebulosa,” writes Dr. Williams, “the tentacula consist of hollow, flattened tubular filaments, furnished with strong muscular parietes, each tentacle forming a band which may be rolled longitudinally into a cylindrical form, so as to inclose a hollow cylindrical space if the two edges of the band meet, or a semi-cylindrical space if they imperfectly meet. This inimitable mechanism enables each filament to take up and firmly grasp, at any point of its length, a molecule of sand, or, if placed in a linear series, a row of molecules. But so perfect is the disposition of the muscular fibres at the extreme free end of each filament that it is gifted with the twofold power of acting on the sucking and the muscular principle. When the tentacle is about to seize an object, the extremity is drawn in, in consequence of the sudden reflux of fluid in its hollow interior; by this movement a cup-shaped cavity is formed, in which the object is securely held by atmospheric pressure; this power is, however, immediately aided by the contraction of the circular muscular fibres. Such, then, are the marvelous instruments by which these peaceful worms construct their habitations, and probably sweep their vicinity for food.”
THERE are many species of Terebella, each remarkable for some peculiarity in the structure of its habitation. On the eastern coast of England, the preceding species seems to be more plentiful, and affords a notable instance of the manner in which stubborn materials can be rendered useful. Some of our coasts, however, are plentifully stocked with the SHELL-BINDER TEREBELLA (Terebella conchilega), a creature which has earned its well-deserved name from its curious habit of choosing broken shells for the construction of its tube. This being the case, it is evident that the creature will be most abundant on those shores where shells are most plentiful, and that it may usually be found on those parts of our coast where the sand is almost wholly composed of shells.
The tube resembles in texture that of the preceding species, but it is rather firmer, owing to the superior size of the material with which it is built. Generally the fragments of shell are very small, no larger, indeed, than those of which the well-known shell-sand is composed. But, in many instances, the Terebella has been known to choose shells in an entire state, and thus to give its tube a very extraordinary aspect.
The process of tube-making is conducted with much rapidity. Supposing that, as is often the case, a storm has arisen, and washed away the projecting portion of the tube, the Terebella sets to work to make a new portion in lieu of that which has been destroyed. Spreading out its long and delicate tentacles in all directions, it brings the materials toward its mouth, and always apportions the number of tentacles employed to the load which is to be carried. Having brought the materials within reach, it arranges them in regular circles, agglutinating them together with some secretion which has the property of hardening under water into a gelatinous substance. The union of these two materials—namely, the gelatinous secretion and the shells which are fastened to it—gives to the tube the two needful qualities of strength and flexibility.
Small particles of shell are generally affixed by the whole of their surface, but the large pieces are only fastened by one edge, so that a tube that is made wholly of large fragments looks as if it were covered with scales, like those of a serpent.
It is a remarkable fact that the Terebella does not form tubes during the early portions of its life, but swims about freely, like the nereis and other marine annelids. It has a head, eyes, feet, and antennæ, and roams about at will; whereas, in its perfect state, it has neither head, nor eyes, nor antennæ, nor true feet, the last-mentioned organs being modified into the tufts of hooks and bristles by means of which it moves up and down its tube. The reader may perhaps remember that the barnacles and many other stationary marine animals are free during their preliminary epochs, and only become fixed when they attain the perfect form. To our minds, the former seems the more perfect, and certainly the more agreeable state of existence; but we can not measure the feelings of such an animal by our own, and may be sure that the creature enjoys existence as much while shut up in a tube as when roaming the ocean at liberty.
ANOTHER species, Terebella figulus, sometimes called the POTTER, prefers mud as the material for its dwelling, and contrives to make the dark sea-mud so adhesive that it is capable of being formed into a tube.
As may be easily imagined, this tube is extremely fragile, and can not be removed entire from the water without the exercise of much care, its own weight being mostly sufficient to tear it asunder. The walls of the tube are tolerably thick, and the tube itself is of some size, measuring nearly half an inch across, and is always found to be protected by the earth upon which it is placed. It is a rather curious fact that the tentacles of this species are of extraordinary length, extending for some eight or nine inches beyond the entrance of the tube, the animal itself measuring little more than four inches in length.
THE last species of Terebella that will be mentioned is a very small and very remarkable species. It has been appropriately termed the WEAVER TEREBELLA (Terebella textrix) from the curious submarine home which it makes.
Not content with using the glutinous secretion as a means for binding together the muddy particles of which the tube is made, it spins a kind of web, bearing some resemblance to that of the spider, and being quite a complicated piece of work. This web is composed of many threads, which are very strong, but are also very fine, and, in consequence, are almost invisible when in the water, and their substance is quite translucent, like the threads of isinglass. The threads encircle the body, and as it is only made in the month of May, when the eggs are deposited, it is in all probability employed more for the sake of guarding the eggs than protecting the body.
The tube of the Weaver Terebella is very small, not sufficing to cover more than half the body. The worm seems to be more independent of its tube than is usually the case, frequently vacating and returning to it, and sometimes making two or three tubes near each other, and living in any of them which it may happen to prefer at the time.
WE now come to a group of tube-building annelids which are called Sabellæ, because they live in the sand, and in most cases form their tubes of that material. The general appearance of the tube is extremely variable. In some cases it bears so great a resemblance to the dwelling of the serpula that a practiced eye is needed to discover the distinction.
One very conspicuous species is the TRUMPET SABELLA (Sabella tubularia), which is generally found attached to stones or shells. The material of which it is made is that hard, calcareous matter which is employed by the serpula, and at first the two tubes seem to be exactly alike. A more detailed examination will, however, show that it is not twisted like that of the serpula, but is nearly straight, looking very much like the military trumpet, or “tuba,” of the ancient Romans. In some cases this tube attains considerable length, measuring eight or nine inches from tip to mouth. It is a solitary animal, and, as far as is yet known, is never found grouped in masses, like many allied species.
The gill-fan of this species is exceedingly beautiful, being white, dotted profusely with scarlet, and expanding into a graceful feathery coronet. Although the resemblance to the serpula is very close, the animal may easily be distinguished by the absence of the beautiful operculum or stopper, which forms so conspicuous a feature in the serpula.
PERHAPS the most plentiful species of this genus is the common SABELLA (Sabella alveolaria), which may be found in countless myriads on many of our coasts. On several sandy shores, especially those of the southern coast, the wanderer by the sea may perceive masses of hard, agglutinated sand, pierced with innumerable holes. These masses are of great size, and in some places are strong enough to bear the pressure of a foot, though in others a slight push with the hand is sufficient to detach a portion.
If this perforated sand be closely examined, it will be seen to consist of a vast number of tubes, which are fixed together, and are farther consolidated by sand which has washed over them, and lodged between them. When the water covers the sand mass, a delicate feathery tuft is seen to protrude from each hole, so that the general aspect is full of beauty. These tufts are the tentacles of the Sabella, and when examined with a microscope of moderate power, each tentacle is seen to be composed of a central shaft, with projecting teeth or fringes on both sides. There are about eighty of these tentacles, and as they are extremely flexible and always in motion, their appearance is peculiarly elegant.
Nothing is easier than to examine the structure of this Sabella, though the task of isolating a single tube is not an easy one. A penknife will soon break up the tube, and a pair of forceps will easily pull out the inhabitant, in spite of the array of bristles and hooks wherewith it clings to its habitation. It is but a little creature in point of length, but in point of width it nearly fills the diameter of the tube. The extremity of the body, however, is very small and slender, and is doubled back upon itself, with its tip pointing to the mouth of the tube.