The form of the Tunicata is irregular. They have two orifices—one at the top, for the entrance of a current of water, and another at one side for its egress. They have two tunics only adhering to one another at the edges of these orifices, which are furnished with a circle of cilia. The irregular or scattered condition of the nerve-centres, as well as the alternation in the circulation, are eminently characteristic of the whole class. They consist of three distinct groups: the compound or social gelatinous Ascidians; the solitary Tunicata, with leathery coats; and the Salpæ, which are gelatinous. The two first, though mobile when young, become fixed when they arrive at maturity; the third floats free on the surface of the ocean.
Fig. 160. Magnified group of Perophora.
The fixed gelatinous Ascidians resemble the Polyzoa in structure and tendency to gemmation; nevertheless, they differ in their circulating and respiratory systems. The Perophora Listeri is an example which is found on the south coast of England and Ireland (fig. 160.) It consists of minute globes of clear jelly, not larger than a pin’s head, spotted with orange and brown, and attached by a foot-stalk to a silvery stem like a thread which stretches over the surface of stones, or twines round the stalks of sea-weeds; and as the stem increases in length, buds spring from it, which in time come to maturity, so that the silvery thread connects a large community; but, though thus connected, every member has its own individuality. Fig. 161 represents one of these transparent individuals very highly magnified.
Fig. 161. Highly magnified Perophora.
The respiratory sac occupies the upper part of the body. It is perforated by four rows of narrow slits, edged with cilia, whose vibrations are distinctly seen through the transparent tunic of the little animal. A portion of the water which is drawn by the cilia into the upper orifice or mouth, passes into the respiratory sac, escapes through the narrow slits into the space between the sac and the tunic, and from thence into the stomach, where any particles of food it may bring are digested, and the refuse is carried by the current through the intestinal canal, and ejected at the lateral orifice.
The heart is a long multiform muscle, attached to the respiratory sac, from whence capillary vessels spread over that sac and throughout the body. The pulsations of the heart drive the blood through the general system, and bring it back to the heart again. After a time the pulses of the heart become faint, and the blood ceases to flow. A short pause takes place, the heart gives an opposite impulse, and the blood makes its circuit in a direction exactly contrary to what it did before. The circulation in all these little globes is brought into connection by a simultaneous circulation through two tubes in the silvery thread to which they are attached.
The average duration of the ebb and flow of the blood is probably the same, but the period between the changes varies from thirty seconds to two minutes. As the blood is colourless and transparent, it probably would have been impossible to determine its motion had it not been for solid particles floating in it.
The larva of the compound sessile Ascidians is like the tadpole of a frog, which swims about for a time; it then fixes itself by the head to some object, the tail falls off, and in a few days it becomes a solitary Ascidian, with its two orifices and currents of water. This solitary animal gives origin by budding to a connected group, which in its turn lays fertilized eggs, so that there is an alternation of generations.
The Botryllidæ or Star-like Ascidians, appear as masses of highly coloured gelatinous matter, spread over stones or fuci in which from ten to twenty minute oblong Ascidians are arranged in a circle round a common open centre which is the discharging orifice of the whole group, for the mouth of each individual is at the opposite extremity. The only indication of life given by this compound creature is the expansion and contraction of an elastic band surrounding the discharging orifice. The organization of each of these individuals is similar to that of the Perophora.
Although many Tunicata form composite societies, the most numerous and largest in size are always solitary, as the Ascidia virginea (fig. 162). Its outer tunic contains cellulose, it is pale and semitransparent, the inner tunic is orange-coloured or crimson. These creatures vary in length from one to six inches: therefore they are not microscopic, yet their internal structure, which is similar to that described, cannot be determined without the aid of that instrument. The organ of hearing is a capsule containing an otolite and coloured spots placed between the orifices; the uppermost orifice or mouth is surrounded by eight eye-specks, and six of a deep orange colour surround the lateral one, a nerve-centre between the two supplies the animal with nerves. These Tunicata live on diatoms and morsels of sea-weeds, and, like all the fixed Ascidians, they show no external sign of vitality except that of opening and shutting the two orifices. More than fifty species of these solitary Ascidians inhabit the British coasts from low-water mark to a depth of more than one hundred fathoms.
Fig. 162. Ascidia virginea.
The Pyrosomidæ are floating compound Ascidians, composed of innumerable individual animals united side by side, and grouped in whorls so as to form a hollow tube or cylinder open at one end only, and from two to fourteen inches long, with a circumference varying from half an inch to three inches. The inhalent orifices of the component animals are all on the exterior of the cylinder, while the exhalent orifices are all on its inside, and the result of so many little currents of water discharged into the cavity is to produce one general outflow which impels the cylinder to float with its closed end foremost. The side of each animal in which the nerve-centre is placed is turned towards the open end of the cylinder, the whole of which is cartilaginous and non-contractile. Each of the Ascidians forming this compound creature has its outer and inner tunic united and lined with a vascular blood system, a respiratory cavity of large size completely enclosed by a quadrangular network, and digesting organs. The sexes are combined, and they are propagated by buds and single eggs. The Pyrosomidæ are gregarious and highly luminous; vast shoals of them extend for miles in the warm latitudes of the Atlantic and Pacific Oceans, and as soon as the shade of night comes on they illuminate ships with bright electric flashes as they cleave the gelatinous mass; half a dozen of these animals give sufficient light to render the adjacent objects visible. The intensity depends upon muscular excitement, for Professor Fritz Müller observed that the greenish blue light of the Pyrosoma Atlantica is given out in a spark by each of the separate individuals; it first appears at the point touched, and then spreads over the whole compound animal. This species appears in such aggregations in the Mediterranean as to clog the nets of the fishermen.
The Salpidæ are another family of free-swimming Ascidians. The tunic is perfectly hyaline, the body is somewhat cylindrical, but compressed and open at both ends (fig. 163). The mouth is a slit, the discharging orifice is tubular and can be opened and shut. The breathing apparatus is in the form of a ribbon extending obliquely across the cavity of the tunic, the ear with four otolites is in the ventral fold, and the flux of the pale blood is alternate as in other Tunicata.
Fig. 163. Salpa maxima.
Fig. 164. Young of Salpa zonaria.
The Salpidæ are produced by alternate generation. A solitary floating Salpa is always found to contain a chain of embryos joined end to end winding spirally within her. They are all of one size, and portions are liberated in succession through an aperture in the tunic. In a little time these connected larvæ are developed into a chain of adult Salpæ. The individuals are from half an inch to several inches long, according to the species, and when joined end to end the chain may extend many feet, but the attachment is so slight that they often break up into shorter portions. The chains swim with an undulating serpentine motion either end foremost by the simultaneous expulsion of water from the muscular tunic of each individual. A single egg is formed by each of these creatures, which remains within the parent till a solitary Salpa is hatched, and then it comes into the water, and after a time produces a chain of larvæ.
The aggregate young of the Salpa zonaria, instead of being united end to end, are applied side to side, and as the individuals are broad at one extremity and narrow at the other, they constitute groups continually diminishing in size, which take a spiral form.
The reproduction of the whole genus of Salpidæ is rapid and enormous. Dr. Wallich mentions that while sailing between the Cape of Good Hope and St. Helena, the ship passed for many miles through water so crowded with the Salpa mucronata that it had the appearance of jelly to apparently a great depth. The Salpæ, which were from one to two inches long, had yellow digestive cavities, about the size of a millet seed, which contained diatoms, Foraminifera, Polycystinæ, small shrimps, and other microscopic creatures.