Part 18

When the Tusk-shell is found alive, we rarely can see more of the soft parts than a sort of white cushion occupying the mouth of the shell, and occasionally protruding or receding, with a little conical point projecting from the centre of it. You might keep it for weeks, as I have done, and see no more, by the most assiduous watching, than this; but at some fortunate moment you might perchance see the whole foot, of which this little cone is the extremity, thrust far out of the cushion-like collar, when you would discern a wide lobed membrane, fringing the base of the foot, trumpet-like in shape, or resembling the blossom of a convolvulus, with the thick and pointed foot projecting from its centre like a pistil.

These sluggish white mollusks ordinarily live on the muddy sea-floor, or burrow in it, where they devour minuter animals, such as Foraminifera, and the spawn and larval forms of their fellow Mollusca. They are rarely taken alive at a less depth than ten fathoms.

TORBAY BONNET.

But we have also an example of a much rarer shell, the Torbay Bonnet, or Cap of Liberty. The shape, which is exactly that of the ancient Phrygian bonnet, or the modern emblem of liberty, is sufficiently commemorated in all the appellations by which it is known, both scientific and popular. This specimen is but an empty shell, but the freshness of the colours, and the beautiful polish of the interior show that the animal cannot have been long dead, for the porcellaneous smoothness and gloss of shells very soon become defaced after their exposure by the death and decay of the soft parts. The interior of this shell is of a most lovely rose-pink, very glossy, and the exterior is nearly of the same hue, though this is concealed by a horny skin which closely invests it, and is covered with a shaggy pile that projects even beyond the edge, in the form of a ragged yellow fringe. This rough epidermis is of a hue varying from a bright yellow-olive to a dull wood-brown; it is frequently rubbed off in the upper parts, when the natural hue of the shell is there seen.

This species is rare enough, and large enough, and handsome enough to be a prize worth finding, when picked up in so fresh a condition as this; but, of course, it is more valuable when it occurs in a living state. But this scarcely ever happens except by dredging, or by trawling. I have frequently had it brought to me by trawlers both at Weymouth and Tenby, oftenest by the former, who get it in deep water, from thirty to fifty fathoms, on the western side of Portland.

The living animal is not unworthy of its elegantly painted house. Its colour is usually pale yellow, with a rose-pink mantle, bordered by a fine orange-coloured fringe. The head is large and swollen, furnished with tentacles, which carry the eyes at their bases. The tongue-ribbon carries seven rows of teeth, of which the central one differs in shape from the rest.

I have kept a specimen in the aquarium for a considerable time, with very little addition to my knowledge as the result. It remained adhering to the scallop shell on which it was found almost all the time I had it, occasionally shifting a hair’s-breadth to the one side or the other. Almost always the fringed edge of the shell was so closely applied to the support as absolutely to forbid intrusion; but now and then a very slight lifting of the edge all round gave me the narrowest possible peep at the broad cream-coloured foot adhering to its rest. Thus it went on tantalizing me, till after some months I lost it, I forget how.

A close ally of this form is a pretty shell which we often obtain by dredging, called the Cup-and-Saucer Limpet. It is a pretty little white porcellaneous cone, with a curved plate of thin shell projecting from the side of the interior, like a semi-cup within the cup. It is named Calyptræa. Both this and the Torbay Bonnet have been ascertained to manifest domestic instincts, in sitting on their eggs till they are hatched. According to MM. Audouin and Milne Edwards, the parent Calyptræa “disposes them under her belly, and preserves them, as it were imprisoned, between the foot and the foreign body to which she adheres, her patelloid shell thus serving not only to cover and protect herself, but as a shield to her offspring. These eggs are oval bodies of a yellow colour, enclosed in membranous capsules, which are elliptical, flattened, translucid, and filled with an albuminous matter. The number of these little capsules varies from six to ten; they are connected among themselves by a footstalk, so as to represent a sort of rosette; each of them contains from eight to ten eggs. It appears that the young Calyptræa are developed under this sort of maternal roof, and do not quit it until they are in a condition to affix themselves, and are provided with a shell sufficiently hard to protect their own bodies.”

LOCUST SCREW.

Along the margins of these shallow rock-pools with sandy bottoms, we see many little grey shrimp-like creatures, with thick-set arching bodies, which swim rapidly, and with much vibration of the abdominal feet, from weed to weed. If we catch one and lay it on the rock or on the wet sand, it appears very helpless, for its vertically thin shape and arched attitude preclude the possibility of its crawling: it falls upon its side, and vainly struggles round and round as on a pivot, or makes aimless jerks by throwing out the tail. It is of a greenish blue, or glaucous colour, marked with red dots. This is the common Locust Screw, which is so confined to sea-water, that it has been affirmed that if put into fresh water it presently dies. But the experiments of Mr. Robertson negative this statement: he has found that five out of seven, after being eighteen hours in rain-water, continued to live upon being returned to salt water.

I delight to trace the manifestations of parental love in these tiny forms of animal life. It had long been known that the females of this species display a solicitude for their infant offspring, which they carry for some time about with them; but the following very pleasing details of maternal manners have recently been observed by my relative, Dr. James Salter, and by him communicated to the learned zoologists who are now engaged upon the history of this order:--

“On catching a female with live larvæ, nothing is seen of the progeny till the parent has become at home in the aquarium, when the little creatures leave her, and swim about in her immediate neighbourhood. The plan I have adopted to watch this curious habit of maternal protection has been to place a single individual in a bottle of sea-water. After a time, and that soon, the little crustacean seems at ease, and swims slowly about, when the young fry leave, and swarm around her in a perfect cloud; they never leave for more than half or three-quarters of an inch, and as she slowly moves about they accompany her. If, now, one taps the sides of the bottle with one’s finger-nail, the swarm of larvæ rush under their parent, and in a second are out of sight. The parent now becomes excited, and swims about quickly, as if trying to escape; but on letting the bottle containing her rest quite still on a table, she soon gets composed, when out come the young larvæ again, and swim about as before. This may be repeated as often as the observer wishes, and always with the same result. I have only seen this in one species, but it is quite a common species in Poole Harbour, and I have watched the interesting habit many times.”

FRESH-WATER SCREW.

Every little stream and ditch of running water in our lanes and fields abounds with a species, which, except for the fresh-water habit, and a little difference in the shade of colour,--the latter being of a yellowish, instead of a bluish grey,--is so absolutely identical with the Locust Screw, that, notwithstanding it is registered under a name of its own, Gammarus pulex, it is difficult to believe that the two are not of common parentage. Certain facts observed in Sweden give colour to such a suspicion. The large inland seas of fresh water, Lakes Wetter and Wener, in the south of that country, are situated on high ground, and have the surface of their water 300 feet above the level of the Baltic, whereas the bottom is 120 feet below such level. In these lakes (which appear to have been lifted up with the gradual uprising of the country) have been found several genera and species of Crustacea, three of which are AMPHIPODA, which are affirmed to be identical with marine ones, viz., Gammaracanthus loricatus (Sabine, Ross, Kröyer), Pontoporeia affinis (Lindström), and Gammarus cancelloides (Gerstfeldt).

The first is now only known to exist in the Arctic seas, the second in the Baltic, and the last was found in Lake Baikal, in Central Asia. It is therefore suggested by Lovèn, that the land was raised so as to convert these waters from marine bays into inland lakes. These marine species were retained within the basins, the waters of which have since been changed, through the agency of springs, into fresh water; and with the gradual transfer of the water, the habits of the animals have also changed gradually, and that without any outward alteration of form. Professor Lovèn thinks that there is sufficient evidence to show that this change in the condition of these lakes must have taken place during the great glacier period, at a time when the animals now found in it (and which are known at this day only to inhabit the extreme north) could have lived in the same latitude as the south of Sweden. The evidence of these fresh-water lakes suggests that similar changes in the relative position of sea and land may have been the cause of our having fresh-water Crustacea nearly allied to marine species in our rivers and inland streams.

Crawling about perpendicular faces of rocks, generally at a considerable distance above high-water mark, we may find a crustacean, not very remotely allied to these, of rather large size. It is broad and flat, a form which marks it as belonging to another order, though in many peculiarities of structure agreeing with its near neighbours the high-backed Screws. The present species crawls readily with its fourteen short strongly-hooked feet, with which it clings to every little roughness of the stone. It swims much less effectively: indeed, I have never seen it spontaneously in the water; and those which I have captured on the rocks, and transferred to an aquarium, have died after a very brief interval. They have little agility under any circumstances, and trust rather to craft than to power for their safety. When alarmed, they instantly gather in their feet, and drop from their hold; and, as I have said that the positions in which they are found are for the most part perpendicular surfaces, such as cliffs and sea-walls, their one trick is often successful.

SEA WOODLOUSE.

Thus the Ligia makes a decided approach to an aërial or terrestrial mode of life; which, indeed, is strictly the habit of some species with which every cultivator of flowers is only too familiar. I refer to the little Woodlice, or Buttons, or Sows, as they are variously called, so abundant in gardens, especially in pits and frames, and so annoying for their depredations on our cherished plants. These garden pests are scarcely to be distinguished as to their structure from the semi-marine Ligia, particularly those flatter and softer species (Porcellio), which do not roll their body into a complete ball.

In all these creatures we find maternal care. The females carry their eggs in a sort of pouch or sac, placed beneath the breast, which opens when these are hatched to afford exit for the infant progeny. These have from the first the same form and appearance as the adult, except that they have at first six pairs of feet instead of seven. They cling about the body of the mother, and are carried by her in her rovings with affecting solicitude.

About the sloping ledges of low-lying rock, especially those which are covered with young mussels, so closely that between their blue-black shells, packed and tied together with byssus-threads, you can scarcely thrust a penknife, we very commonly see slender Worms of a vivid grass-green hue gliding in and out. Their movements are very graceful, and if we examine them closely, we discern the presence of a multitude of minute points along each side of the body, which in turn are thrust out and as quickly retracted. If we put the little creature into a vase of water, we see fresh powers and organs of locomotion; for the back bears a double series of leaf-shaped fins, overlying one another, which act as broadoars rowing the lithe and graceful worm through the sea.

RAINBOW LEAF-WORM.

This species is not more than three inches in length; but there is another much less common, which is larger, richer in colours, and altogether a finer species. A specimen which I lately found was about a foot in length when crawling, and nearly a quarter of an inch in width. The whole body was divided into distinct segments, which in this individual were about two hundred and ten in number; though, according to MM. Audouin and Milne Edwards, they sometimes amount to nearly five hundred, and the worm has been found two feet in length. The head is small, terminated by two pairs of very minute antennæ, but surrounded at what we may call the neck by four pairs of rather long feelers (tentacular cirri), with a fifth pair which are minute and rudimentary.

PLATE 18.

P. H. GOSSE, del. LEIGHTON, BROS.

PEARLY NEREIS. RAINBOW LEAF-WORM. ]

The colours are very beautiful. The leaf-like fins of the sides, which are somewhat heart-shaped, are of a yellowish green hue, occasionally clouded with blackish; the middle portion of the back, which is exposed, is of a rich brown, but flushed with the most glowing iridescence of blue and purple; while the whole under parts are of a pearly flesh-colour. As it crawls over the stones, it throws its body into the most elegant lateral curves, while its suppleness and great length cause it to cling close to the rock; and thus its outline takes the form of every projection and depression over which it is wandering.

When disturbed, and often without any apparent provocation, we see the under side of the tiny head rise from the ground, swell out, and turn itself inside out, as you turn a stocking, until a great pear-shaped bag is protruded, fully eight times as long, and thrice as broad as the entire head. Its whole surface is rough and papillose, and around its extremity, which is the largest part, there is a row of small knobs or warts. This curious organ is found under similar conditions in very many Annelids; it is commonly called the proboscis, or evertile œsophagus; but in truth it is a special and peculiar apparatus, with little analogy with anything found in higher animals: it is in some species furnished with strong horny teeth, and is doubtless employed for the capture of living prey, and the conveyance of it to the stomach.

The tail of the specimen I refer to was evidently being renewed after having been accidentally lost. For the body-segments ended abruptly, and were followed by a portion, not more than an eighth of an inch long, white, and excessively delicate; but which, when examined with a powerful magnifier, displayed a division into segments, each segment carrying its proper cirri; I could count twenty segments within that minute space, the last of which carried the usual pair of stylets.