A highly curious item of sea-squirt history is furnished at the outset by the consideration of the rough bag or ‘test’ in which its organs are inclosed. When the chemist analyses this part of the animal, he finds it to be composed in greater part of a substance known as _cellulose_. It so happens that cellulose is a most important constituent of plants, being almost as common in vegetables as starch. Hence zoologists accounted it a strange and unwonted proceeding on the part of an animal, that it should manufacture in a seemingly natural manner a substance proper and peculiar to the plant-world. The multiplication of cases of like kind in animals has destroyed the novelty and unique nature of the sea-squirt’s case; but none the less curious must the fact be accounted, that the animal should mimic the plant in the mode and results of its life. When the tough outer sac is cut open, we come upon a much more delicate and softer structure, known as the _mantle_. This latter forms an inner lining to the test, and is the structure upon the presence of which the sea-squirt’s power of ejecting water depends. The mantle is a highly muscular layer, and lies next the organs and internal belongings of the animal. The clearest mode of describing the structure of the sea-squirt is that of beginning with that neck of the bottle-shaped body on which the mouth opens. The mouth leads, curiously enough, not into a throat, but into a large chamber, named the breathing-sac. The walls of this chamber may be simply described as composed of a network of fine blood-vessels; the meshes of this network being provided with those delicate vibratile filaments, named cilia, the function of which is to keep up, by their movements, a constant circulation of the water admitted to the breathing-chamber. Just within the mouth-opening a few small tentacles or feelers exist, these organs serving to guard the entrance to the body. On the floor of the breathing-sac an opening may be perceived; this aperture leading into the throat, and being, therefore, by many naturalists termed the true mouth. And in the way of digestive apparatus, we find the sea-squirt to possess a stomach, intestine, and other organs.