Hypsibius exemplaris
Observations upon the central nervous system of a single individual, with measurements not previously taken.
The animal in lateral aspect, the body rendered transparent, showing the brain (above, right) and the four ventral ganglia with their paired connectives. Reconstructed from X-ray tomography of one adult.
Length ....... 152 µm
Method ....... synchrotron nano-tomography, 0.27 µm per voxel
Collected by . Gross, Müller, Hehn, Ferstl, Allner, Dierolf, Achterhold, Mayer, Pfeiffer
Published .... 1919 → corrected: 2019, Zoological Letters 5:14
Licence ...... CC BY 4.0 — which is why this sheet exists at all
Upon the brain, and the four ganglia
The brain of this animal occupies one thousand three hundred and eighty-nine cubic micrometres, which is a single hundredth part of the whole creature. It sits over the gut like a saddle, wrapped about the tube through which the animal feeds.
Behind it, beneath the gut, lie four swellings of the nerve cord, one to each pair of legs. The authors who published this scan state plainly that they did not measure these. We have therefore done so, by severing the cord wherever its cross-section exceeds ten square micrometres — a threshold at which the connectives (two and a half to six) are never mistaken for the swellings (forty-seven to fifty-five).
The volumes so obtained are set down opposite. The fourth ganglion is half the bulk of its fellows, and remains half however the cut is made.
| Part | Volume | Relative to ganglion I |
|---|---|---|
| Ganglion I | 385 µm³ | 1.00 |
| Ganglion II | 430 µm³ | 1.12 |
| Ganglion III | 379 µm³ | 0.98 |
| Ganglion IV | 194 µm³ | 0.50 |
| The four together | 1,386 µm³ | — |
| The brain | 1,389 µm³ | — |
| Connectives between | 214 µm³ | — |
That the four ganglia should come to the volume of the brain within three parts in a thousand is a thing we report without explanation, being unwilling to furnish one.
Upon the order in which it is built
Sixty-two embryos, each killed and its transcripts read at a known hour between laying and hatching. The genes that lay out the nervous system are found to rise together between the forty-first and fifty-third hours — twelve hours in a development of four days — and thereafter to fall silent.
Within that same window a further gene appears which is absent from nearly every earlier embryo, increasing two hundred and eighty-eight fold. It is the counterpart in this animal of unc-86, by which a nematode determines which of its cells shall become which of its neurons. No gene of this family has before been reported in any tardigrade.
* The reader is cautioned. This is one animal, dried; and the expression is read from whole embryos, so that it cannot say in which cells anything occurs. The ganglion volumes depend upon a threshold, whose range of tolerance we have given. And that a gene is the counterpart of another is a reason to look further, not a finding in itself.