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higher and more elaborate ancestors. These are certain marine animals, the Ascidians, or sea-squirts. These animals are found encrusting rocks, stones, and weeds on the sea-bottom. Sometimes they are solitary (Fig. 13), but many of them produce buds, like plants, and so form compound masses or sheets of individuals all connected and continuous with one another, like the buds on a creeping plant (Fig. 14).

We will examine one of the simple forms—a tough mass like a leather bottle with two openings; water is continually passing in at the one and out at the other of these apertures. If we remove the leathery outer case (Fig. 15), we find that there is a soft creature within which has the following parts: Leading from the mouth a great throat, followed by an intestine. The throat is perforated by innumerable slits, through which the water passes into a chamber--the cloaca : in passing, the water aërates the blood which circulates in the framework of the slits. The intestine takes a sharp bend, which causes it to open also into the cloaca. Between the orifice of the mouth and of the cloaca there is a nerve-ganglion.

My object in the next place is to show that the structure and life - history of these Ascidians may be best explained on the hypothesis that they are instances of degeneration; that they are the modified descendants of animals of higher, that is, more elaborate structure, and in fact are degenerate Vertebrata, standing in the same relation to fishes, frogs, and men, as do the barnacles to shrimps, crabs, and lobsters.

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The young of some, but by no means of all these Ascidians, have a form totally different from that of their parents. The egg of Phallusia gives rise to a tadpole, a drawing of which placed side by side with the somewhat larger tadpole of the common frog is seen in the adjoining figure (Fig. 16). The young Ascidian has the same general shape as the young frog, but not only this; the resemblance extends into details, the internal organs agreeing closely in the two cases. Further still, as shown by the beautiful researches of the Russian naturalist, Kowalewsky, the

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FIG. 16.-Tadpole of Frog and of Ascidian. Surface view.

resemblance reaches absolute identity when we examine the way in which the various organs arise from the primitive egg-cell. Tail, body, spiracle, eye, and mouth agree in the two tadpoles, the only important difference being in the position of the two mouths and in the fact that the Ascidian has one eye while the frog has two.

Now let us look at the internal organs (Fig. 17). There are four structures, which are all four possessed at some time of their lives by all those animals which we call the Vertebrata, the great branch of the pedi

gree to which fishes, reptiles, birds, beasts, and men belong. And the combination of these marks or structural peculiarities is an overwhelming piece of evidence in favour of the supposition that the creatures which possess this combination are derived from one common ancestor. Just as one would conclude that a man whom one might meet, say on Salisbury Plain, must belong to the New Zealand race, if it were found not only that he had the colour, and the hair, and the shape of head of a New Zealander, but also that he

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FIG. 17.-Tadpole of Frog and of Ascidian. Diagrams representing the chief internal organs.

was tattooed like a New Zealander, carried the weapons of a New Zealander, and, over and above, in addition to these proofs, that he talked the Maori language and none other; so here, in the case of the vertebrate race, there are certain qualities and possessions, the accumulation of which cannot be conceived of as occurring in any animal but one belonging to that race. These four great structural features are-first, the primitive backbone or notochord; second, the throat perforated by gill-slits; third, the tubular nerve-centre or spinal cord and brain placed along the back; and lastly, and

perhaps most distinctive and clinching as an evidence of affinity, the myelonic or cerebral eye.

Now let us convince ourselves that these four features exist not only in the frog's tadpole, as they do in all fishes, reptiles, birds, and beasts, but that they also exist in the Ascidian tadpole, and, it may be added, coexist in no other animals at all.

The corresponding parts are named in Figs. 16 and 17, in such a way as to render their agreement tolerably clear, whilst in Fig. 18 a more detailed representation of the head of an Ascidian tadpole is given.

ch

C

N

K

FIG. 18.-Ascidian Tadpole with a part only of the tail C. N, nervous system with the enlarged brain in front and the narrow spinal chord behind (n); N', is placed in the cavity of the brain; 0, the single cerebral eye lying in the brain; a, similarly placed auditory organ; K, pharynx; d, intestine; o, rudiment of the mouth; ch, notochord or primitive backbone. (From Gegenbaur's Elements of Comparative Anatomy.)

The

It is clear then that the Ascidians must be admitted to be Vertebrates, and must be classified in that great sub-kingdom or branch of the animal pedigree. Ascidian tadpole is very unlike its parent the Ascidian, and has to go through a process of degeneration in order to arrive at the adult structure. The diagrams which are reproduced in Figs. 19 and 20 show how this degeneration proceeds. It will be observed, that

in somewhat the same manner as the young barnacle, young Ascidian fixes itself to a stone by its head;

the

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FIG. 19.-Degeneration of Ascidian Tadpole to form the adult. The black pieces represent the rock or stone to which the Tadpole has fixed its head.

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FIG. 20.-Very young Ascidian with only two gill-slits. Compare with Fig. 15; which is, however, seen from the other side, so that left there corresponds to right here.

then the tail with its notochord and nerve-chord atrophies. The body grows and gradually changes its

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