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.
Pyrosomidæ.
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.