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Updated: June 5, 2025
The ontogenetic facts that we gather from this sole survivor of the Acrania are the more valuable for phylogenetic purposes, as paleontology, unfortunately, throws no light whatever on the origin of the Vertebrates. Their invertebrate ancestors were soft organisms without skeleton, and thus incapable of fossilisation, as is still the case with the lowest vertebrates the Acrania and Cyclostoma.
The upper division of this group consists in great part of beds of compact, fragile limestone, with some intercalated green marls. The shells in some parts are a mixture of Cerithium, Cyclostoma, and Corbula; in others Limnea, Cerithium, Paludina, etc. In the latter, the bones of reptiles and mammalia, Palaeotherium and Lophiodon, have been found.
A short search among the damp stones and moss brought to light some small but interesting landshells, consisting of a pupiform Cyclostoma, a Carocolla, and five species of Helix.
In this way the central marrow of the Vertebrates divides clearly into its two chief sections, brain and spinal cord. But in these it soon passes away, the one vesicle being divided into several successive parts by transverse constrictions. In all the Craniotes, from the Cyclostoma up to man, the same parts develop from these five original cerebral vesicles, though in very different ways.
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