The breathing-chamber, as we have seen, receives fresh sea-water from the outside world, this water containing the vivifying oxygen required for the purification and renewal of the blood. Having given up its oxygen to the blood contained in the fine blood-vessels of the breathing-chamber, and its sediment to serve for food, the great bulk of water contained in the breathing-sac has now to be got rid of, to make room for a fresh supply. This process is effected in the most admirable manner through the currents created by the little filaments or cilia, which cause a constant flow of water to pass through the network walls of the breathing-chamber into a second sac or bag which lies parallel with it. This latter sac receives the name of the _atrium_, and communicates with the outer world by the second neck or orifice of the body. Hence the water which enters by the mouth-opening, after passing through the breathing-chamber, is ejected by the second aperture of the body, and affords the material wherewith the sea-squirt vents its indignation on prying humanity in the shape of the _jets d’eau_ which have procured for it its popular designation. The sea-squirt regarded in relation to its sedentary habits, would seem to require no great exercise of nervous powers. Accordingly we find its nervous system to be represented by a single mass of nervous matter, placed near the mouth, and from which nerves pass to the other parts of the body. The acts of a sea-squirt may thus be regarded as purely of the character we are accustomed to name ‘automatic.’ It is provided with instincts enabling it to carry on the acts of its life and to exhibit a certain degree of irritability, without at the same time knowing the ‘reason why’ of its own actions.