Cyclostoma

The class that is formed of the Myxinoides and Petromyzontes is called the Cyclostoma (round-mouthed), because their mouth has a circular or semi-circular aperture.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

The presence of an operculum seems to have been of importance, as out of twelve specimens of Cyclostoma elegans, which is thus furnished, eleven revived.

XIII. Geographical Distribution-Continued. Absence of Batrachians and Terrestrial Mammals on Oceanic Islands 1909

Cyclostoma, the oldest fishes, amphibia, mammals (not including man).

The Evolution of Man — Volume 1 Ernst Heinrich Philipp August Haeckel 1876

The jaws (upper and lower) that we find in all the higher Vertebrates are completely wanting in the Cyclostoma, as in the Amphioxus.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

One of these lines is preserved in a greatly modified condition: these are the Cyclostoma, a very backward and partly degenerate side-line.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

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.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

These five primitive cerebral vesicles, that are found in the embryos of all the higher vertebrates from the fishes to man, and grow into very complex structures, remain at a very rudimentary stage in the Cyclostoma.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

Cyclostoma are usually classified with the fishes.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876

The Cyclostoma are "one-nosed" (Monorhina); their nose is a single passage in the middle of the frontal surface.

The Evolution of Man — Volume 2 Ernst Heinrich Philipp August Haeckel 1876