The older zoologists used to speak of Vertebrata and Invertebrata as animals with a back-bone and animals without one, and everyone thought it a very natural way of dividing up the animal kingdom. It never occurred to anyone that it was possible to bridge the interval between them and find a link between the two. But now the Vertebrata have been compelled to give up their aristocratic pretensions, and own that they have risen from the ranks of the common people of the animal world; in other words, that they are descended from the Invertebrates. Their family secrets have been published to the world, and now everybody knows that they have poor relations. But how many, and how nearly related? This we do not accurately know; consequently the whole zoological world for many years has concentrated all its energies on attempts to find out the truth about the matter.

A great sensation was caused by the first discovery of a poor relation of the vertebrates among the Ascidians, or Leather-bottle animals. These are named from their shape and texture, for they have a leathery skin. Now some of these Ascidians have larvÆ with a tail; and in the tail there is a long cord-like structure, which in many essential particulars resembles the cord which precedes the back-bone in the vertebrate embryo. This structure is called the Notochord (a string down the back). The credit of this great discovery belongs to Russia; for the presence of the Notochord in the Ascidian larva was discovered by A. Kowalevsky, in 1866.

To the present generation of zoological students, the Chordate affinities of Ascidians are part of the ABC of knowledge; and it is hardly possible for them to realise that it is only thirty years ago since the idea was so new that Huxley, in his "Text-book of the Vertebrata," only alluded to it in a footnote. Would-be zoological critics, at a somewhat later period, met the theory with ridicule, for want of better argument. For critics include not only "those who have failed in literature and art," but also those who have failed in science.

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.

Some of the Ascidians present what has been already described in other types (p. 57) as "alternation of generations." The discovery of this fact was made by the poet Adelbert von Chamisso. Some of his verses are known to English readers, for whom they were translated by Mary Howitt, a poetess whose writings were popular with our grandmothers, and deserved to be so. This is not the only case in which a poet has been also a zoologist: Goethe studied the science, and framed a theory regarding the vertebrate skull, which he regarded as consisting of a series of vertebrÆ. In this he was less fortunate than the Italian poet; for while Chamisso's observations were correct, and were confirmed by subsequent writers, Goethe's theory of the skull is anything but correct. It was made worse, too, by the speculations of subsequent writers, who attempted to follow it into detail, with the result of demonstrating its absurdity.