By the deep sea

Look at these diagrams: they represent two common forms of Ascidians, and it will be noted that they have a general agreement in shape with the large specimens of A. mentula we were looking at just now. Like that, these have each two necks, though those of mentula were closed, and these are open at their mouths. If we had these in a glass vessel, but still attached to pieces of the rock upon which they grew, we should be able to see why one bottle need have two necks. If we were then to drop a little finely-divided colour-powder such as indigo, into the water, we should see two currents were in operation, one flowing to the animal, the other proceeding from it. The first would be flowing to the neck marked a in the figures, and the second would be issuing from the mouth of b. Naturally, we should at once suppose that by means of some internal mechanism and system of valves, the same current that was being induced at a, was being continued through the creature’s body, and pumped out at b. Our supposition would be proved correct by the fact that the colour grains streaming in were also streaming out. But what happens to them between entering and departing we cannot clearly see.

Now in order to get a correct idea of the Tunicates—as the group in which the Ascidians are included is called—I wish you to note the figures d and e in the same diagram. You will see that they indicate two separate envelopes. The outer of these, represented by the thick dark line, is of a tough, leathery nature, and is much akin to vegetable cellulose in its character—a fact that caused some little commotion in scientific circles years ago, when it was first satisfactorily made out, for prior to that date cellulose was considered to be purely a vegetable product. This outer coat is known as the tunic, or test, and from the fact that all the species are enclosed in such a tunic, the group gets its name Tunicata. The inner coat represented in the diagram by the clear space between the thick and thin marginal lines, is of a more delicate, more animal nature: it is composed of soft though powerful muscular tissue, and by its contraction the water, which always fills the interior of this “leather bottle,” can be violently spurted forth—a phenomenon which has brought upon these creatures the name of Sea-squirts. This muscular coat is known as the mantle.

The Ascidian has no proper system of blood-vessels, as we are generally acquainted with them in higher animals. The blood flows about the general body cavity, and is not confined to narrow channels as in our arteries and veins. There is a heart, it is true, but one of the simplest character, without any elaborate system of ventricles and auricles, with their regulating valves. The Ascidian’s heart is simply a tube open at each end, and by its steady pulsation—that is, its alternate contraction and expansion, it sets the blood flowing to the blood-vessels that line the walls of the branchial cavity, where it absorbs oxygen from the continuous flow of fresh sea-water that passes through it. When this end has been attained, a curious and unique “reversal of the engine” takes place: there is such an opposite action of the heart, that all this vivified blood is withdrawn from the neighbourhood of the branchial chamber and sent flowing to remote parts of the body.

The flow of water through the branchial chamber is kept up by the regular and unceasing lashing of eye-lash-like cilia, with which the blood-vessels are fringed. This constant inflow at the oral orifice forces the water through to the atrial chamber, from which it is pumped out by the contraction of the mantle. Minute particles of matter that serve as food are also brought in by the current, and find their way into special grooves for their reception and digestion. The other arrangements of the creature are equally simple. The nervous ganglion, to which we have made reference, is its only brain, and it has no proper eyes, only some pigment granules near the tentacles appear to be sensitive to light.

Most of the Ascidians inhabit deeper water than comes within our range, but we shall find specimens at low-water attached to stones and the roots of seaweeds. We may even find specimens of Ascidia mentula in rock-pools, and others we shall discover on smaller stones and shells that have washed in on sandy shores from greater depths. Among such will be the Quadrangular-squirter (Cynthia quadrangularis), so-called on account of the squareness of its apertures; and the Currant-squirter (Styela grossularia), a very common form on dead shells, which gets its name partly from its colour and partly from its form when it has closed both apertures and become more rounded.

But there are many other forms of Tunicates that haunt our shores either in deep water or upon the vegetation of the lower rocks. There are some of more slender, more elongated form that live together in bunches, their bases being connected by a kind of running rootstock, which has the power to produce young individuals by budding from it. This form is known as Clavelina lepadiformis, and is only about an inch in height, of the form shown in the annexed diagram. In the figure the reference letters are of the following signification: a, branchial apertures; b, atrial apertures; c, young individuals arising from the runners s.

From this form it is an easy transition to the Ascidians known as Salpæ. These have the branchial aperture (b) at one end, and the atrial opening (a) at the other. In the figure the heart is shown at e, and the branchial chamber at d. These Salpæ are both solitary and compound Ascidians, for it is a singular fact that the solitary form as here shown produces buds which develop into a connected series or chain of individuals. These, in turn, instead of reproducing the species, in a similar manner produce eggs, each of which gives rise to a solitary individual. In our figure of Salpa maxima, the letters c indicate the points of attachment of the Salpa colony; and the next figure represents a portion of the Salpa-chain.

Frequently, in gazing down the sides of a still, deep rock-pool, we shall observe a coating of dark grey jelly, in patches as big as one’s hand, and on carefully taking off an inch or two of this, and examining it with the lens, or a low power of the microscope, we shall observe a number of elegantly-formed jars to be set in the jelly, and as we look their mouths and necks are raised above the surface of the jelly and opened. These are the branchial apertures of a colony of Ascidians (Leptoclinum gelatinosum), and if we search around the mass we shall shortly find a cone-shaped opening in the clear jelly, through which a current of water flows. This is the common atrial chamber of the whole colony. The clear jelly is the common outer tunic of the whole community.

On the walls of overhanging rocks, at low-water, many fleshy clusters, like pale-coloured strawberries will be found, of firm gelatinous material, with a clear jelly envelope, through which the crimson dots of the contained squirts may be seen. One form has a thick trunk, with but slightly enlarged head, and consisting of a number of groups of squirts: this is Aplidium; it has no common aperture. A more globose head on a shorter stalk has a distinctly marked common opening: this is Polyclinum. Amaroecium has a corrugated exterior, and is more cylindrical in form.

Other species will meet us of more symmetrical form, on flat weeds, smooth stones, and under the overhanging brows of the large rocks at low-water. These are of varied tints according to species, but each with a starry pattern worked in with little purple or yellow Ascidians. It looks as though six or seven of these had agreed to live together for company’s sake and for economy; and here we find them set in the jelly, and radiating from a central aperture, the common atrial opening of the colony.

Here is a figure showing part of a patch of Botryllus violaceus, such as you may find abundant on the rocks. C shows the combined tunic of the colony, a the branchial openings, and b the common atrium. The general verdict on a patch of Botryllus would probably be that it was some low form of sea-plant, for a naked-eye view of it reveals no evidence of animal processes; yet, in spite of its vegetative condition, this—in common with other Tunicates—is held to approach nearest to the great back-boned races, the aristocracy of animal life.

But it is a sad story of missed opportunities and consequent degeneration that the Tunicates have to tell of their race. Some evolutionists hold that in the primeval Ascidian we must look for the progenitor of the vertebrates. We know what the primeval Ascidian was like, for the form is retained, according to a natural law, in the larval stage of its present-day representatives. Roughly speaking, it was like a tadpole, with a broad head-and-trunk combined, and a very long, narrow tail, by the lashing of which from side to side it made way through the waters, much as the boatman gets along by sculling from the stern. At the front there was a rudimentary mouth with three suckers, an optic organ, with a retina, lens, cornea, and so forth; an auditory organ; the promise of a well-formed brain and nervous system; and a rod in the tail might be developed into that backbone which is the distinguishing mark of all the birds, beasts, fishes, reptiles, and man himself.

Some of the primeval squirt-larvæ are supposed to have cultivated these possibilities, and the grand vertebrate division of the animal kingdom is said to be the result; but others went in for the status quo and inglorious ease. No developments for us, said they. They may even be supposed to have anticipated the prayer formerly taught to rural school children:

“God bless the squire and his relations,
And keep us in our proper stations.”

Then they gave up wandering at random through the waters, and settled down to a quiet and retired life on a piece of rock at the root of a branching weed. Taking hold with their suckers, they soon discovered that tails and sense organs were of no use to those who had forsworn wandering, so they threw them off, and gradually assumed the wine-skin shape that has ever since been the ruling fashion among Ascidians. All that remains of the tail is a few fatty cells in the posterior part of the trunk. The suckers by which it was attached disappear, and the test grows over surrounding objects; the auditory organ disappears, the eye retrogrades into a mere pigment spot, and the nervous system degenerates into the solitary ganglion to which we have already referred. It will thus be seen that the life-history of the Tunicates is a dismal story of degeneration instead of development; but it is none the less interesting on that account.