Are echinoderms vertebrates
Youth in secret
By Dagmar Röhrlich
The graceful sea lilies with their flexible stems and flower-shaped tentacles are among the oldest living things on earth. They bring it to more than 500 million years. But neither fossils nor living animals in the sea have so far succeeded in finding out anything about the childhood of these echinoderms, these echinoderms. How the life of sea lilies begins was unknown, while other echinoderms such as starfish, sea urchins and sea cucumbers have been very well researched. But researchers are particularly interested in the larval stage of sea lilies, because they are the most primitive representatives of this group. Thurston Lacalli, of the University of Victoria in British Columbia, Canada, describes what the Japanese researchers from the University of Tokyo saw in their laboratory:
The Japanese researchers found that the embryos and larvae of the sea lilies develop like those of other echinoderms. Just like with the other echinoderms, the larvae of the sea lilies initially have eyelash bands that are shaped like an ear. In contrast to this larval phase of starfish or sea cucumbers, the sea lily larvae do not eat at this stage. However, since the morphology of these larvae is exactly the same, we suspect that the sea lilies once had a feeding first larval stage, so that their "Spartan" way of life is a secondary development. This means that the form of the feeding larva goes back a long way into the evolution of the echinoderm.
After six to ten days, the young sea lily - just like its millimeter-sized cousins of the other echinoderms - goes through an astonishing change. The eyelash bands, which were previously arranged in the shape of an ear, break open and regroup into rings within a short time. This shape is also typical for all young echinoderms. In this development phase, the animals can swim comparatively quickly. It is the time when the "permanent" species find a place to begin their sedentary lifestyle. The sea lilies as the most primitive representatives share their larval forms with all other echinoderms - and that provides an insight into the work of evolution. Lacalli:
That's nice for the echinoderms, you might say, but what does that mean for me? The special significance lies in the fact that the echinoderms are the closest relatives of the vertebrates among the invertebrates. More than 500 million years ago, the ancestors of the sea lilies seem to have separated from those of the so-called chordates, which also include vertebrates. It is known from molecular and morphological studies that the embryonic stages of both echinoderms and chordates have properties in common. So from an evolutionary point of view it is very interesting to know what the earliest larval stages look like in the oldest and most primitive form of echinoderms.
The adult animals of sea lily and fish, for example, are not at all similar. But in their development they have similarities that differentiate them from the rest of the animal world: the ciliate bands of the echinoderm larvae are said to have been the starting point for the development of the spinal cord in the so-called chordata, which also includes vertebrates. These eyelash bands are "operated" by nerves, and the nerve cords of the spinal cord are said to have developed from them. While this is a controversial hypothesis and there is little evidence to support it, there is no alternative either. No other known invertebrates have suitable structures to serve as a starting point for the chord data. With the larvae of the sea lilies, we could take a look far back into the history of the development of life in our day.
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