Part 19
The action of the heart in these transparent creatures is equally visible. Below the muslin curtain with its living chambers, down at the very bottom of the body-cavity, there is a transparent sac of membrane, which takes the appearance of a long bag, pointed at each end, but not closed, and strangely twisted on its long axis, so as to make three turns. This is the heart; and within it are seen many colourless globules, floating freely in a clear fluid, which answers to the blood. This circulates throughout the system in the following manner:--We see a spasmodic contraction at one end of the bag, which drives forward the globules contained there; the contraction in an instant passes onward along the three twists of the vessel, the part behind expanding immediately as the movement passes on, and the globules are forcibly expelled through the narrow but open extremity. Meanwhile the free globules surrounding the commencing end have rushed in as soon as that part resumed its usual width, and are in their turn driven forward by the periodic repetition of the pulsation. The fluid, with its globules thus put in motion, is then driven along through the interstices of the various organs of the body, not through a system of closed blood-vessels, some finding their way along the transverse lines that separate the rows of gill-ovals, until they sooner or later arrive at the point where they entered the heart, to take the same course over again.
As in the kindred forms of animal life, the same orifice, the same cilia, the same currents are subservient to breathing and to the reception of food; the stomach digesting the microscopic animalcules which are poured with the entering stream through the receiving siphon. At some distance within the interior of this orifice there are a series of thread-shaped tentacles, affixed in a ring, which we may suppose to exercise some kind of superintendence, by touch or other perception, over the atoms which indiscriminately enter upon the stream, accepting or rejecting. Probably it is in the exercise of the latter discretion that those irregular regurgitations of the current take place, accompanied by a momentary closing of the mouth, that we frequently notice.
THE EYES.
Still further ancillary to the protection of the stomach from the intrusion of inimical matters, we may safely suppose certain eye-like specks which are placed at the very vestibule. In the larger species, as this red-clouded green Squirter, there are seated in special fissures at the very margin of the expanded siphon-orifices, red dots,--eight around the receiving, six around the ejecting, siphon. Each dot seems ascertained to be an eye of very rudimentary structure, seated on a mass of orange pigment. We should probably do wrong if we attributed any higher vision to these organs than a low degree of sensibility to the general stimulus of the light.
Some species have the orifices of the siphons four-cornered, whereas the sort I have been describing have them circular; there are differences also in the breathing sac, which in the square-mouthed species is folded lengthwise, while in the round-mouthed it is plain. Hence the former have been separated from the _Ascidiæ_, as a distinct genus, named _Cynthia_; both including a large number of species.
We have a pretty attractive little _Cynthia_ in our dredging,--the Currant Squirter.[152] It is not uncommon in deep water off this coast, and in Weymouth Bay; frequently occurring in family groups crowded together on old shells. Generally there is one of superior dimensions to the rest, the venerable parent of the colony; and, surrounding him, others of varying size, and (doubtless) age, down to very minute infants. The full size is about that of half a small cherry; but it is more usual to see them not exceeding that of half a red currant. These comparisons will also give a fair idea of their shape and colour, especially in a state of contraction, as when lifted from the water: for they are little hemispheres of a brilliant, scarcely pellucid, crimson hue, seated on the shell by the whole broad base. Under water, and at ease, the form becomes more conical, rising into a point; whence, in full expansion, the two siphons protrude, slightly divergent, and the one a little superior to the other, each opening by a distinctly quadrangular orifice.
FOUR-ANGLED SQUIRTER.
Contrasting with this neat and pretty little family, we have here another species of the genus, the Four-angled Squirter.[153] You would hardly suppose this to be an animal at all, if uninitiated; but might readily pass it over as a rude stone, or a bit of wood roughly bruised and worn by the waves beating it among the rocks, so uncouth and coarse and shapeless it is. It forms a great mass, some two inches high, rudely four-sided, of a dull yellowish-olive hue, rising into two blunt eminences, which individually retain the quadrangular shape, and in activity open by symmetrically quadrangular orifices. It is a sluggish, unattractive lump of flesh, somewhat between leather and jelly in texture, coarsely pellucid, but not transparent, and its exterior is usually distinguished by various extraneous matters imbedded in the test, as well as by forests of tangled zoophytes which creep over it and root in it as on the rock. The surface itself, moreover, is much corrugated by an irregular network of depressions, marking off angular warty areas.
On the other hand, the little Currant is a pleasing inhabitant of the aquarium. Of manners, to be sure, it has not much, good or bad, but the form and colour are agreeable; as is also the effect produced by the grouping of the brilliant drops of jelly. Little of change takes place, beyond the occasional contraction and reprotrusion of the orifices; but sometimes you may see, as I have seen, at certain times, the laying of eggs by this species, which is an interesting phenomenon. These are perfectly globular, about the size of small shot or pins’ heads, of a rich scarlet-crimson hue: they are deposited in a singular manner. The oviduct does not extend to the exterior of the body, but discharges the eggs into a large cavity formed by the mouth, of which the discharging siphon is the outlet. From this orifice, then, they are expelled, shot out perpendicularly with considerable force, so that they describe arched courses through the water, like bombs shot from a mortar, rising to a height ten times that of the animal. I have observed a dozen or more eggs thus discharged in quick succession, which then fall to the bottom around the parent, destined to constitute one of those family groups in which we usually find the species.
I have not been successful in rearing these eggs to maturity. The development of the _Tunicata_ has, however, been observed by various naturalists, and by none more thoroughly than by the late Sir John Dalyell, whose elaborate and costly works, profusely illustrated, are such a mine of information respecting the lower forms of marine life.
TRANSFORMATIONS.
The young escapes from the egg in a form as unlike the parent as can be imagined. It is a flat ovate body with a long flat tail, altogether presenting a curious resemblance to the tadpole of a frog. In this condition I have found the larva of the _Clavelina_, and have followed it to the development of the _Ascidia_ form. By means of the rapid vibrations of the powerful tail, the little tadpole swims for short distances through the water, with more effort than effect. After a while, it rests; swims again, and again rests; till at last it moves no more. A coloured eye-speck is visible on the surface, destined to be absorbed; the tail is beginning to disappear (in some cases it is separated by a spontaneous constriction at its junction with the body, in others it appears to be gradually absorbed); one or more warts are seen budding from the opposite extremity of the body. These last secrete a cement by which the animal is finally attached to its support, shell, stone, or sea-weed, either growing out into creeping and adherent root-threads, or enlarging into a broad base, from which the body begins to grow upward. After a while the two orifices are formed; first within, on the mantle, before the exterior test is pierced; then the internal organs, the gill-sac, and the pulsating heart, if it be one of the transparent species, become recognisable; the single eye-speck, a temporary organ, pales and disappears; and the permanent circles of visual organs are formed around the siphonal orifices. And thus the Ascidian is developed.
BOTRYLLI.
The genera _Ascidia_ and _Cynthia_ consist of isolated distinct individuals; the _Clavelina_ and the _Perophora_ exist in the form of groups, composed of distinct individuals associated by a common branching root-thread, whence they irregularly bud forth. There are, however, other genera, in which the compound life is more prominently manifest, the individual being recognisable only by carefully dissecting it out from the common mass. To this form belong many gelatinous masses which occur on our rocky coasts; one in particular,[154] conspicuous for its rich scarlet and orange colours, which forms irregular pear-shaped lumps, that hang from low-lying ledges, and that look not unlike strawberries. This is extremely abundant. More attractive still are the _Botrylli_,[155] which doubtless many persons have gazed on with admiration, wondering what they can be,--animal, vegetable, or what. They look as if small quantities of jelly had been spilt, sometimes on a stone, sometimes on the broad leaf of a tangle, sometimes on a shrubby sea-weed, entangling the twigs and leaves in the gelatinous mass. When we look closely at such an object, we see that it is studded with little starry systems of oblong specks, of some bright colour contrasting with the ground-tint;--perhaps the stars are bright orange on a warm brown, or pale straw-colour on a chocolate ground, or green on an iron-grey. The stars vary in outline, often being angular, often oval, or circular: they vary, too, in dimensions, and in number of the constituent specks; some may be a sixth of an inch in diameter, and contain a dozen or more; others may be less than half that size, and have no more than two or three; or even a single speck may be seen here and there, which has not yet begun to develop the starry form.
Each bright speck in these radiating star-like systems is an animal essentially of the Ascidian form, with the following peculiarities. All have budded from one primary individual, which was produced as a tiny tadpole, from an egg: the manner and direction in which the buds were put forth determining the starry arrangement. There is a common gelatinous envelope, in which the whole are imbedded, and which ever extends as the individuals and systems multiply, and which seems to have the power of developing isolated individuals which have not budded in the ordinary way, but which then produce others by budding, and so become the commencing points of other systems. In each individual the siphonal orifices are remote from each other, the receiving one being placed on the circumference of the ring or system, while the ejecting one is placed at the opposite end, opening, in common with the discharging siphons of all of that system, into a central main orifice, which rises out of the level with a circular rim, and forms the dark centre of the system.
In the open ocean there are forms of _Tunicata_ which are not attached, but swim freely; and, what is surprising, even compound forms are thus found, progressing by a combined action. One of the most curious is the genus _Pyrosoma_, which consists of long-bodied Ascidians, so united in rings as to constitute a long, free cylindrical tube, closed at one end and open at the other. By the rhythmical contractions and dilatations of the multitude, this great cylinder slowly swims through the open sea. But the most interesting circumstance in its history is that it is intensely luminous, lighting up the midnight ocean with flashes of vivid light, or seen gliding through the dark water like glowing sticks of fire. So it is described by some voyagers, perhaps with a little exaggeration, for Mr. Bennett, to whom we are indebted for many valuable remarks on oceanic zoology, speaks in more subdued tones of it. His account of this and other pelagic phosphorescence is very interesting:--
PYROSOMA.
“On the 8th of June,” he observes, “being then in lat. 30° S., and long. 27° 5′ W., having fine weather and a fresh south-easterly trade-wind, and the range of the thermometer being from 78° to 84°, late at night, the mate of the watch came and called me to witness a very unusual appearance in the water, which he, on first seeing it, considered to be breakers. On arriving upon deck, this was found to be a very broad and extensive sheet of phosphorescence, extending in a direction from east to west, as far as the eye could reach. The luminosity was confined to the range of animals in this shoal, for there was no similar light in any other direction. I immediately cast the towing-net over the stern of the ship, as we approached nearer the luminous streak, to ascertain the cause of this extraordinary and so limited phenomenon. The ship soon cleaved through the brilliant mass, from which, by the disturbance, strong flashes of light were emitted; and the shoal, judging from the time the vessel took in passing through the mass, may have been a mile in breadth. The passage of the vessel through them increased the light around to a far stronger degree, illuminating the ship. On taking in the towing-net, it was found half filled with _Pyrosoma_ (_Atlanticum?_), which shone with a beautiful pale-greenish light; and there were also a few shell-fish in the net at the same time. After the mass had been passed through, the light was still seen astern, until it became invisible in the distance; and the whole of the ocean then became hidden in the darkness as before this took place. The scene was as novel as beautiful and interesting; more so from my having ascertained, by capturing luminous animals, the cause of the phenomenon.
“The second occasion of my meeting these creatures was not exactly similar to the preceding; but though also limited, was curious, as occurring in a high latitude, during the winter season. It was on the 19th of August, the weather dark and gloomy, with light breezes from north-north-east, in lat. 40° 30′ S., and long. 138° 3′ E., being then distant about 368 miles from King’s Island (at the western entrance of Bass’s Straits). It was about eight o’clock P.M. when the ship’s wake was perceived to be luminous; and scintillations of the same light were also abundant around. As this was unusual, and had not been seen before, and it occasionally, also, appeared in larger and smaller detached masses, giving out a high degree of brilliancy, to ascertain the cause, so unusual in high latitudes during the winter season, I threw the towing-net overboard, and in twenty minutes succeeded in capturing several _Pyrosomata_, giving out their usual pale-green light; and it was, no doubt, detached groups of these animals that were the occasion of the light in question. The beautiful light given out by these molluscous animals soon ceased to be seen emitted from every part of their bodies; but by moving them about it could be reproduced for some length of time after. As long as the luminosity of the ocean was visible (which continued most part of the night), a number of _Pyrosoma Atlanticum_, two species of _Phyllosoma_, an animal apparently allied to _Leptocephalus_, as well as several crustaceous animals (all of which I had before considered as intertropical species), were caught and preserved. At half-past ten P.M. the temperature of the atmosphere on deck was 52°, and that of the water 51½°. The luminosity of the water gradually decreased during the night, and towards morning was no longer seen, nor on any subsequent night.”[156]
SPONGES.
Let us come back from the wide world of waters, with its nightly illuminations, and its other ten thousand marvels, to our own homely and quiet beach. A tribe of existences is awaiting our notice, which we have as yet neglected; which yet we cannot fail to observe whenever we peep beneath these boulders, and look at these rocky ledges, just left exposed by the lowest retiring tides. They are the Sponges; the most debateable forms of life, long denied a right to stand in the animal ranks at all, and even still admitted there doubtingly and grudgingly by some excellent naturalists. Yet such they certainly are, established beyond reasonable controversy as true and proper examples of animal life, and therefore having a rightful claim to be painted and described in this series of essays. However, as they are the lowest, so shall they be the last; for, with a few notes on some of our species, I shall dismiss my kind and courteous readers.
The inferior surface of this huge slab of limestone, supported at one end by a boulder, while the other end is imbedded in the mud or concealed by the smaller fragments that are thrown in confusion around, is densely studded with organisms. It is only at very low tides that this arch is exposed, though now we can creep in and work with no great discomfort, though with some defilement of our garments from the mud and slime. The absence of direct light is favourable to the growth of marine productions, and thus we have another congenial element in the obscurity. With some of the smaller sea-weeds, chiefly of the filamentous kinds, as _Cladophora_, _Conferva_, some of the minuter _Callithamnia_, etc., the majority of the forms that crowd and cover the rock so densely are animals: indeed there are large areas where the animate forms struggle so perseveringly for standing-room, that not a filament or frond of vegetation can be seen, and you could not thrust the point of a penknife down to the rock in any spot without wounding some or other of the incrusting creatures. _Polyzoa_ are here, chiefly of the tufted species; _Hydrozoa_, too, hang down; Anemones, but not in abundance, may be seen; _Saxicavæ_ push their crimson siphons through, here and there; _Botrylli_ are spread in patches, and _Amœrœcia_ are suspended like a plentiful crop of tempting strawberries; but Sponges constitute the staple of the crop; it is a veritable field of Sponge.
I have on other occasions described some of the more characteristic phenomena of this class of creatures;--the volcanic eruptions of the Crumb-of-bread Sponge, the hills and poles and webs of the Rosy Crumb, the protrusile bladders of the Sanguine, the starry spicula of the Flat-Sack.[157] These I shall assume as known, and shall confine myself to the enumeration and description of a few other species which are found congregated on this rocky roof.
One of the first to catch the eye, by its gorgeous colour, is a rather thin, soft, spreading patch, of the richest vermilion hue.[158] It is of close substance, the surface covered with shallow irregular sinuous channels and minute orifices. It rarely exceeds an inch in diameter, but throws out slender clinging processes to some length. The flesh is dense. Under the microscope it contains three-rayed spicula, which are for the most part somewhat blunt, but some are very sharp. After death it rapidly loses its brilliant hue, and dries of a dull oak-brown.
Another occurs in the form of low irregular spreading patches of a greyish black, very smooth and shiny, more plump than the former, and a little larger.[159] There are no visible orifices on the surface. The substance is yellowish grey, compact, with a distinct demarcation from the thin black tough skin. The spicula of the flesh have three rays in one plane, and one standing up from it; very stout, sharp-pointed, with a distinct canal running through the centre of each ray of the larger ones. There are a few rods abruptly hooked at the tip; a few very long, straight, and slender, with pin-like heads; and a few simple needles, long, slender, pointed at one or at both ends.
Some largish rounded masses occur, several inches in diameter, and rising to a thickness of three-fourths of an inch.[160] The surface is undulate, the summits slightly ridged, covered with shallow sinuous channels having sharp edges, not very confluent: large round mouth-orifices (_oscula_) generally mark the summits of the ridges. The colour of this fine species is a deep buff, and its appearance is much like that of our Turkey Sponge, with something of the same feel, but much firmer. The interior is full of channels. The spicula here are very varied and interesting, comprising some curious and unusual forms. Some, indeed, are simple needles, nearly straight, pointed at one end rather abruptly. The following are minute. There are a number of rings interrupted at one side, like the letter C, some thicker and more elliptical, others slender and rounder, C. A few take the form of the letter S, but having one lobe much more developed than the other. Some of the C form have both the extremities bifid, and the points widely diverging; and some of these have the back straight, instead of curved, and these with their bifid points, look, when viewed in front, like double anchors. All are, as usual, spun out of the most brilliant glass.
Then we have in some abundance a sort[161] with firm cartilaginous walls, about one-eighth of an inch thick, standing up to the height of three-fourths, with rounded summits, running in irregular sinuations and convolutions, not unlike the cartilage of the human ear, enclosing deep hollows. The colour is opaque white, tinged with red, probably from the red mud, which is prevalent here. When cut with a knife this sponge has a sharp, crisp, gritty feel, and when a little is crushed between glass plates we distinctly hear a creaking sound. These phenomena depend on the circumstance that it is almost wholly composed of great stout three-rayed spicula, solid throughout, together with a multitude of excessively slender needles, straight, long, pointed at one end; and many others as slender, but very short; no longer, indeed, than the thickness of one of the three-rayed.[162]
Equally numerous with these, and possessing a certain amount of resemblance to them, are some thick, compact, sack-shaped masses, with angular edges and blunt points: sometimes they are flattened and dilated, like a sack when empty; sometimes rounded, like a sack when full. In the former condition several perforations occur along the terminal ridge, in the latter there is generally but one. The colour is white, slightly tinted, just as in the preceding. It stands up boldly and stiffly (or rather _hangs_ in the natural state) from its rather narrow base to the height of about two-thirds of an inch, and the flattened specimens are as wide as this. It is nearly made up of three-rayed spicula, some of large size, but more rather small. A transverse section shows no obvious arrangement, except that of the great central channel, into which the points of the spicula project; but a longitudinal section shows the spicula built up one on another in many courses, so nearly symmetrical that hexagonal canals are formed, whose axes run transversely to the axis of the sponge; _i.e._, horizontally. There is very little fleshy or gelatinous matter.[163]
Again we see a showy species,[164] making soft, spongy patches of an orange or red-lead colour, an inch or more in diameter, rising into ridges a quarter of an inch high, and forming low peaks, whose apices are perforate. Its substance contains simple needles, long, nearly or quite straight, pointed at one end; these are found in great numbers in close array, the points mostly projecting from the surface. Some of them are twice as thick as others.