Aphrodita, or Sea-Mouse.
In most of the wandering annelides, each segment is provided with variously formed appendages, more or less developed, serving for respiration and locomotion, or for aggression and defence; while in some of the least perfect of the class, not a trace of an external organ is to be found over the whole body. The great Band-worm (Nemertes gigas) is one of the most remarkable examples of this low type of annelism. It is from thirty to forty feet long, about half an inch broad, flat like a ribbon, of brown or violet colour, and smooth and shining like lackered leather. Among the loose stones, or in the hollows of the rocks, where he principally lives on Anomiæ,—minute shells that attach themselves to submarine bodies,—this giant worm forms a thousand seemingly inextricable knots, which he is continually unravelling and tying. When after having devoured all the food within his reach, or from some other cause, he desires to shift his quarters, he stretches out a long dark-coloured ribbon, surmounted by a head like that of a snake, but without its wide mouth or dangerous fangs. The eye of the observer sees no contraction of the muscles, no apparent cause or instrument of locomotion; but the microscope teaches us that the Nemertes glides along by help of the minute vibratory ciliæ with which his whole body is covered. He hesitates, he tries here and there, until at last, and often at a distance of fifteen or twenty feet, he finds a stone to his taste; whereupon he slowly unrolls his length to convey himself to his new resting place, and while the entangled folds are unravelling themselves at one end, they form a new Gordian knot at the other. All the organs of this worm are uncommonly simplified; the mouth is a scarce visible circular opening, and the intestinal canal ends in a blind sack.
Nature has not in vain provided the more perfect annelides with the bristly feet, which have been denied to the Nemertes and the sand-worm. Almost all of them feed on a living prey,—Planarias and other minute creatures—which they enclasp and transpierce with those formidable weapons. Some, lying in wait, dart upon their victims as they heedlessly swim by, seize them with their jaws, and stifle them in their deadly embrace; others, of a more lively nature, seek them among the thickets of corallines, millepores and algæ, and arrest them quickly ere they can vanish in the sand.
But the annelides also are liable to many persecutions. The fishes are perpetually at war with them; and when an imprudent annelide quits its hidden lurking-place, or is uncovered by the motion of the waves, it may reckon itself fortunate, indeed, if it escapes the greedy teeth of an eel or a flat-fish. It is even affirmed of the latter, as it is of the whelks, that they know perfectly well how to dig the annelides out of the sand. The sea-spiders, lobsters, and other crustacea are the more dangerous, as their hard shells render them perfectly invulnerable by the bristling weapons of the annelides.
While the greater part of these worms lead a vagrant life, others, like secluded hermits, dwell in self-constructed retreats which they never leave. Their cells, which they begin to form very soon after having left the egg, and which they afterwards continue extending and widening according to the exigencies of their growth, generally consist of a hard calcareous mass; but sometimes they are leathery or parchment-like tubes, secreted by the skin of the animal, not however forming, as in the mollusks, an integral part of the body, but remaining quite unconnected with it. Thus these tubicole annelides spend their whole life within doors, only now and then peeping out of their prison with the front part of their head.
As they lead so different a life from their roaming relations, their internal structure is very different, for where is the being whose organisation does not perfectly harmonise with his wants? Thus, we find here no bristling feet or lateral respiratory appendages; but instead of these organs, which in this case would be completely useless, we find the head surmounted by a beautiful crown of feathery tentaculæ, which equally serve for breathing and the seizing of a passing prey. Completely closed at the inferior extremity, the tube shows us at its upper end a round opening, the only window through which our hermit can peep into the world, seize his food, and refresh his blood by exposing his floating branchiæ to the vivifying influence of the water.
Do not, therefore, reproach him with vanity or curiosity, if you see him so often protrude his magnificently decorated head; but rejoice rather that this habit, to which necessity obliges him, gives you a better opportunity for closer observation. Place only a shell or stone covered with serpulas or cymospiras, into a vessel filled with sea-water, and you will soon see how, in every tube, a small round cover is cautiously raised, which hitherto hermetically closed the entrance, and prevented you from prying into the interior. The door is open, and soon the inmate makes his appearance. You now perceive small buds, here dark violet or carmine, there blue or orange, or variously striped. See how they grow, and gradually expand their splendid boughs! They are true flowers that open before your eye, but flowers much more perfect than those which adorn your garden, as they are endowed with voluntary motion and animal life.
Serpula, attached to a Shell.
At the least shock, at the least vibration of the water, the splendid tufts contract, vanish with the rapidity of lightning, and hide themselves in their stony dwellings, where, under cover of the protecting lid, they bid defiance to their enemies.