In this case there are not pairs of limbs, and it is the same with the oldest craniate animals, the cyclostoma (myxinoida and petromyzonta).
Within the ranks of the vertebrates we find to-day a long series of evolutionary stages which reach up from the lowest acrania and cyclostoma to the fishes and dipneusta, from these to the amphibia, and from these again to the amniota.
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.
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.
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.
But apart from these peculiarities the Cyclostomadiffer more widely from the fishes in other special features of their structure than the fishes do from man.
It has descended from the Cyclostoma by a profound degeneration, and these in turn from the fishes; even the Ascidia and the whole of the Tunicates are merely degenerate fishes!
To mention only the chief points, the Cyclostoma show no trace of pairs of limbs.
In most other respects the organisation of the Cyclostoma is much simpler--for instance, in the structure of the heart, circulation, and kidneys.
The brain of the Cyclostoma is merely a very small and comparatively insignificant swelling of the spinal marrow, a simple vesicle at first.
These Cyclostoma are usually classified with the fishes.
The few surviving species of the Cyclostoma are divided into two orders--the Myxinoides and the Petromyzontes.
Unequal segmentation takes place in some of the cyclostoma and in the oldest fishes in just the same way as in most of the amphibia.
But it is only in the amphioxus and the cyclostoma that the axial rod retains its simplest form throughout life.
In some of the fishes (selachii) and in the cyclostoma we find six or seven of them permanently.
Among the cyclostoma ("round-mouthed") the familiar lampreys are particularly interesting.
In the cyclostomaand the frog these features are, on the whole, not much altered cenogenetically, but they are very much so in the chick, and most of all in the rabbit.
We have an interesting intermediate stage between the acrania and the fishes in these and many other respects in the cyclostoma (the hag and the lamprey, cf.
The distinguishing character of this genus is the contraction of the last whorl near the aperture, in which respect it is nearly resembled by the shell called Cyclostoma lucidum.
Cyclostoma and others are large, filling up the cavity at its outer edge.
However, the pouch gills and the round sucking mouth of the Cyclostomamust probably be looked upon as purely adaptive characteristics, which did not exist in the corresponding stage of ancestors.
Now, strange as it sounds to say so, it is pretty certain that we must really class this lung-breathing cyclostoma among the gill-breathers, because of its close resemblance to the paludina.
Besides, we have many intermediate links between the pond-snails and my cyclostoma here.
The fact of its occurrence in the cyclostoma would thus be quite inexplicable if we were compelled to regard it as a descendant of the other lung-breathing mollusks.
But the cyclostoma is a snail which introduces one to still deeper questions.
But besides its beauty, the cyclostoma has a strong claim upon our attention because of its curious history.
Achatina Eulina, and Cyclostoma lucidum (the first three of which are extinct throughout the entire group), seem to have altogether disappeared.
We have here positive evidence that such shells as Helix pomatia and Cyclostoma elegans might easily be transported to an island from the mainland.
The presence of an operculum seems to have been of importance, as out of twelve specimens of Cyclostoma elegans which it thus furnished, eleven revived.
The above list will hopefully give you a few useful examples demonstrating the appropriate usage of "cyclostoma" in a variety of sentences. We hope that you will now be able to make sentences using this word.
