The majority of the Ascidians are sessile animals, which fix themselves, like Sea-Anemones, to some object when they have passed their earliest stages of growth; and although there are many forms that swim freely, most authorities are inclined to believe that these have arisen by adaptation, and that the kinds that are fixed when adult are the original type of the group. Anything more unlike what we should expect to find as a relative of the vertebrates could not possibly be imagined. What has been written about these little animals by various observers would make a whole series of volumes of the size of this one, so many are the puzzles afforded by their internal structure. The arrangement of their organs is in many respects very unsymmetrical. Their most striking peculiarity, perhaps, is the nature of the gills. These form a kind of basket-work, consisting of minute holes with intermediate supports; and they are associated with a special cavity outside them called the Atrial chamber. Into this the gills pass the sea-water which they have breathed. The group, as a whole, is sometimes considered to present evidence of having degenerated from a higher type; but whatever else may be doubtful or obscure in its history, the nature of the larval notochord is quite clear and certain; zoologists have never had any doubt about its nature since the first few years after its discovery. Ascidians are not at all uncommon animals on the English coast. Some of them may be met with on stones near low-water mark, and I have often seen them on the shells of oysters sold in the shops--for there the town-dwelling naturalist may often find a good many interesting things without much trouble. They are like little lumps of tough jelly; of various colours, according to the kind, red being the most common, and of very indefinite shape. You may see some of the colonial kinds forming pretty star-shaped patterns, attached to various objects, such as stones and the larger seaweeds. The place of Ascidians in classification was a puzzle, until their relationship with Vertebrates was discovered. At one time they were placed with the Mollusca. Now they are grouped, together with the Vertebrata and some other creatures that remain to be spoken of, under the name of Chordata, or animals possessing a Notochord.