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.