Vietnam or Thailand ? Vote for the TOP Country of the Week !
Updated: May 15, 2025
The two small arrows indicate the entrance and exit of the water through the openings of the mantle. The hind end, which corresponds to the tail of the Amphioxus, is usually attached, often by means of regular roots. The dorsal and ventral sides differ a good deal internally, but frequently cannot be distinguished externally.
Hence the formation of a medullary tube out of the outer skin takes place in the naked dorsal surface of the free-swimming larva of the Amphioxus in just the same way as we have found in the embryo of man and the higher animals within the foetal membranes.
Thus, for instance, in the development of the human embryo, the transition from the invertebrate to the vertebrate may be represented in the reality by the isolated amphioxus, which remains stationary where vertebrate man begins, and can make no step forward, while the human embryo advances farther and farther till it reaches its highest limit.
It is true that the Amphioxus is far below all other living vertebrates. It is true that it has no separate head, no developed brain or skull, the characteristic feature of the other vertebrates. Every single organ in it is simpler and less advanced than in any of the others. Yet the characteristic connection and arrangement of all the organs is just the same as in the other vertebrates.
In the Amphioxus the spinal marrow continues to develop, but in the Ascidia the tube soon shrinks into a small and insignificant nervous ganglion that lies above the mouth and the gill-crate, and is in accord with the extremely slight mental power of the animal.
Immediately under the thin corium, and closely connected with it, we find a thin muscle tube, as in the worms. On the other hand, the Ascidia has a centralised heart, and in this respect it seems to be more advanced than the Amphioxus. On the ventral side of the gut, some distance behind the gill-crate, there is a spindle-shaped heart.
All these processes, which outline the typical structure of the vertebrate, take place with astonishing rapidity in the embryo of the Amphioxus; in the afternoon of the first day, or twenty-four hours after fertilisation, the young vertebrate, the typical embryo, is formed; it then has, as a rule, six to eight somites.
Here stagnation and degeneration mean, as a rule, extinction. Of all the relatives of vertebrates back to worms only the very aberrant lines of amphioxus and of the tunicata remain. Of the rest not a single survivor has yet been discovered. And yet what hosts of species must have peopled the sea. The primitive round-mouthed fishes have practically disappeared.
In the middle, between the two lateral coelom-folds of the primitive gut, a single central organ detaches from this at an early stage in the middle line of its dorsal wall. This axial rod, which is the first foundation of the later vertebral column in all the vertebrates, and is the only representative of it in the Amphioxus, originates from the entoderm.
It requires no technical education to appreciate the value of this to the original investigator, particularly to the student of life problems. A skilful worker may do much with a single specimen, as, for example, Johannes Mûller did half a century ago with the one available specimen of amphioxus, the lowest of vertebrates, then recently discovered.
Word Of The Day
Others Looking