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In these the auscultory organ always contains three circular canals, but in the lampreys there are only two, and in the hag-fishes only one. In most other respects the organisation of the Cyclostoma is much simpler for instance, in the structure of the heart, circulation, and kidneys.
It seems that this habit was very widespread among the earlier Vertebrates; the larvae of many of the Ganoids and frogs have suctorial disks near the mouth. But apart from these peculiarities the Cyclostoma differ more widely from the fishes in other special features of their structure than the fishes do from man.
A very rudimentary skull is developed at the foremost end of their chorda. The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first. It afterwards divides into five successive cerebral vesicles, like the brain of the Gnathostoma.
Oldest vertebrates without jaws or pairs of limbs, single nose. : 13. Acrania II. More recent. : 13. Leptocardia. Amphioxus. Oldest vertebrates without jaws or pairs of limbs, single nose. : 14. Petromyzonta larvae. Oldest vertebrates without jaws or pairs of limbs, single nose. : 15. Cyclostoma II. More recent. : 15. Marsipobranchia. Petromyzonta. Silurian. : 16. Selachii. Primitive fishes.
The human brain, seen from below. But comparative anatomy and ontogeny teach us that in man and all the other Craniotes the brain is at first composed, not of these two, but of three, and afterwards five, consecutive parts. These are found in just the same form as five consecutive vesicles in the embryo of all the Craniotes, from the Cyclostoma and fishes to man.
While the Amphioxus is thus connected through the Cyclostoma with the fishes, and so with the series of the higher vertebrates, it is, on the other hand, very closely related to a lowly invertebrate marine animal, from which it seems to be entirely remote at first glance.
The brain from above, v fore brain, z intermediate brain, m middle brain, h hind brain, n after brain. The brain with the uppermost part of the cord, from the left. In the Cyclostoma a stage above the Acrania the fore end of the cylindrical medullary tube begins early to expand into a pear-shaped vesicle; this is the first outline of an independent brain.
Some of the marine shells may have lived on the spot; but the Cyclostoma and Limnea, being land and fresh-water shells, must have been brought thither by rivers and currents, and the quantity of triturated shells implies considerable movement in the waters. There occur no less than 137 species of this genus in the Paris basin, and almost all of them in the calcaire grossier.
There are to-day only a few species of this once extensive class, and these may be distributed in two groups. One group comprises the hag-fishes or Myxinoides. The other group are the Petromyzontes, or lampreys, which are a familiar delicacy in their marine form. These Cyclostoma are usually classified with the fishes.
We must especially note the absence of a very important organ that we find in the fishes, the floating-bladder, from which the lungs of the higher Vertebrates have been developed. One of these lines is preserved in a greatly modified condition: these are the Cyclostoma, a very backward and partly degenerate side-line.
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