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.