CHAPTER XIV.

CLASS TUNICATA.

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THE animals known as Tunicata are all inhabitants of the sea, and derive their name from the fact that the body is enclosed in a sort of bag or "tunic." They are also often called Ascidians (Greek, askos, a wine-skin), from the fact that many of them have a close resemblance to a leathern wine-skin or bottle. Rarely," remarks Prof. Edward Forbes, "is the dredge drawn up from any sea-bed at all prolific in submarine creatures without containing few or many irregularly-shaped leathery bodies, fixed to sea-weed, rock, or shell, by one extremity or by one side, free at the other, and presenting two more or less prominent orifices, from which, on the slightest pressure, the sea-water is ejected with great force. On the sea-shore, when the tide is out, we find similar bodies attached to the under surface of rough stones. They are variously, often splendidly, coloured; but otherwise are unattractive, or even repulsive, in aspect. These creatures are Ascidia, properly so called." As the type of this group we may take Ascidia (Phallusia) mentula, a not very rare inhabitant of British seas; but for various reasons it will be sufficient to indicate very briefly the chief points in its anatomy.

Ascidia mentula (fig. 21, A) presents itself in the form of an oval or oblong, leathery body, which is attached by the greater portion of one side to a stone or to a shell, and which has one extremity drawn out into two prominent necks, each perforated by a distinct opening. The animal thus has very much the appearance of a twonecked jar. If allowed to rest in a basin of sea-water, it can be seen that a current of water is drawn in. by the longest or highest of the two necks of the sac or jar, and is expelled again from the lower neck. If the animal be

rudely touched, it contracts itself forcibly, and squirts out a jet of water from both necks.

Fig. 21.-A, Ascidia (Phallusia) mentula. Two individuals attached to the inner surface of an Oyster-shell, natural size. B, Diagrammatic section of the same: a Opening by which the water enters; b Opening by which the water escapes; p Respiratory chamber; g Gullet; s Stomach; i Intestine; v Vent; c Chamber into which the water escapes after passing through the respiratory sac; h Heart; t Outer layer of the integument; t Inner muscular layer or "tunic."

The outer covering of the body (fig. 21, B, t) is of a leathery nature, but is nevertheless partially translucent, especially in the neighbourhood of the necks. It is remarkable for containing a substance apparently identical with the woody fibre of vegetables. This outer covering is loosely lined by an inner layer or "tunic" (t') which is of a highly muscular nature, and which confers upon the animal its power of squirting out water.

The highest of the two necks of the animal (fig. 21, B, a) is perforated by an opening surrounded by eight lobes, at the bases of which is a series of rudimentary eyes, in the form of yellow spots, each with a red point in its

centre. This opening leads into a great chamber, which may be termed the "respiratory sac (fig. 21, B, p), and which has a reticulated aspect, owing to its being perforated by numerous small apertures. The water which the animal takes in at a passes into this respiratory sac, and then escapes through the openings in its walls into a second chamber or sac, which opens on the surface at the other neck (b). In this way the creature both breathes and obtains its food. From the bottom of the perforated respiratory sac proceeds a gullet (g) which opens into a stomach (s), this in turn leading to an intestine (¿), which terminates in a distinct vent (v) at the base of the second chamber.

There is a distinct heart (h) in the form of a tube open at both ends, and alternately propelling the blood in opposite directions. The nervous system is in the form of a little nervous mass (n) placed between the two necks.

Ascidia mentula attains a length of from one and a half to three inches, and may be obtained by dredging in from ten to thirty fathoms' depth. It can readily be kept alive with a little care; and it is sufficiently transparent to allow of the easy observation of the water-currents which constitute such a striking feature in its vital functions.

RECAPITULATION OF ESSENTIAL CHARACTERS.-Body furnished with two openings, conducting into two chambers which occupy the greater portion of the cavity of the animal. Internal organs enclosed in a double sac, the outermost layer of which is more or less leathery, whilst the inner is muscular. The nervous system is in the form of a single nervous mass. There is an alimentary canal, and a distinct heart. Respiration effected by currents of water which enter at one of the openings of the sac and are expelled from the other. These characters distinguish the Tunicata as a whole.

CHAPTER XV.

CLASS BRACHIOPODA.

THIS class comprises a number of shell-fish which agree with one another in having the body enclosed within a double ("bivalve") shell, and in having two long spiral processes, or "arms," attached to the sides of the mouth. From this last-mentioned peculiarity the name of the class is derived (Greek, brachion, arm; podes, feet). Owing to the great rarity of the few forms of Brachiopods which inhabit British seas, a foreign representative of the group has been selected-viz., Terebratula flavescens of the seas of Australia; and a brief description will suffice to indicate its leading peculiarities.

The body of Terebratula flavescens is soft, and the internal organs are enclosed in a modification of the integument, which is termed the "mantle." The front and back portions of the mantle produce a shell, which is composed of carbonate of lime, and conceals the animal within it. The shell (fig. 22, A) is double, or, in other words, consists of two distinct pieces, which are called "valves," so that the shell is said to be "bivalve " (Latin, bis, twice; valve, folding-doors). The entire shell has somewhat of the shape of an antique Roman lamp, each valve having a prominence or "beak" at one extremity, and one of them having this beak perforated by a round aperture (ƒ), similar to the hole through which the wick passes into the lamp. (Hence the Brachiopoda are sometimes called "lamp-shells.")

The two valves of the shell are jointed together at the "beaks" by means of interlocking teeth and sockets, and they are of decidedly different sizes. The smaller valve lies over the back of the animal, and is not perforated by any aperture. Internally, however, it carries a singular shelly loop (fig. 22, B, 7), which supports the

bases of the spiral "arms," and which is sometimes called the "carriage-spring-apparatus." The larger valve lies

Fig. 22.-Terebratula (Waldheimia) flavescens. A, The shell viewed from behind, showing the smaller valve, and the perforated summit of the larger valve above it. B, Inner view of the smaller valve, showing the shelly loop (1) which supports the spiral arms. C, Inner view of the larger valve, showing the foramen or aperture (f) in the beak, through which the muscular stalk of attachment passes. D, Longitudinal and vertical section of the animal, showing the spiral arms (a), the stomach (s), and the liver (h). At fis the opening in the beak, with the stalk of attachment (p) passing through it. After Davidson and Owen. Some details have been omitted in figs. B, C, and D, for the sake of clearness.

upon the lower surface of the animal, and has its beak perforated by a large rounded aperture (fig. 22, C, ƒ). Through this opening passes a muscular stalk, by which the shell is firmly fixed to some foreign body (fig. 22, D, P).

The animal contained within this shell exhibits many peculiarities of structure, but none is more striking than