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These early ancestors of man, thus seen in the dim recesses of time, must have been as simply, or even still more simply organised than the lancelet or amphioxus. There is one other point deserving a fuller notice.
II, § 1 and § 2 we have already referred to the value which Darwin, and more especially Häckel, lays on the relationship of the larva of the ascidia to the lancelet fish. Now the important testimony of K. E. von Baer, in his "Mémoires de l'Académie de St.
When the ova of the female and the sperm of the male are ripe, they fall into the atrium, pass through the gill-clefts into the fore-gut, and are ejected through the mouth. Transverse section of the lancelet, in the fore half. Above the sexual glands, at the dorsal angle of the atrium, we find the kidneys. Transverse section through the middle of the Amphioxus.
This fish is remarkable for its negative characters; it can hardly be said to possess a brain, vertebral column, or heart, etc.; so that it was classed by the older naturalists amongst the worms. Many years ago Prof. Goodsir perceived that the lancelet presented some affinities with the Ascidians, which are invertebrate, hermaphrodite, marine creatures permanently attached to a support.
In the transition that once took place from one species of ascidian larva to a form similar to the lancelet fish, he sees the new branch diverging in the series of vertebrates.
In order to understand it properly, we must first consider these remarkable animals in their fully-developed forms and compare their anatomy. We begin with the lancelet after man the most important and interesting of all animals. Man is at the highest summit, the lancelet at the lowest root, of the vertebrate stem.
The sea-squirt has in its early youth the line of cartilage through the body which, in embryonic development, represents the first stage of the backbone; the lancelet and the Appendicularia have a rod of cartilage throughout life; the "acorn-headed worm" shows traces of it.
When we compare this embryonic condition, the sandal-shaped foetus, with the developed lancelet, we may say that the amphioxus is, in a certain sense, a permanent sandal-embryo, or a permanent embryonic form of the Acrania; it never rises above a low grade of development which we have long since passed. The body is pointed at both ends, but much compressed at the sides. There is no trace of limbs.
These animals probably gave rise to a group of fishes, as lowly organised as the lancelet; and from these the Ganoids, and other fishes like the Lepidosiren, must have been developed. From such fish a very small advance would carry us on to the Amphibians.
Haeckel, E., on the origin of man; on rudimentary characters; on death caused by inflammation of the vermiform appendage; on the canine teeth in man; on the steps by which man became a biped; on man as a member of the Catarrhine group; on the position of the Lemuridae; on the genealogy of the Mammalia; on the lancelet; on the transparency of pelagic animals; on the musical powers of women.
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