United States or Ecuador ? Vote for the TOP Country of the Week !
Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.
From the first gill-arch, from the inner surface of which the muscular tongue proceeds, we get the first structure of the maxillary skeleton the upper and lower jaws, which surround the mouth and support the teeth. These important parts are wholly wanting in the two lowest classes of Vertebrates, the Acrania and Cyclostoma.
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 comparative anatomy of these fins is very interesting, because they contain the rudiments of all the skeletal parts that form the framework of the fore and hind legs in all the higher vertebrates right up to man. There is no trace of these pairs of limbs in the Acrania and Cyclostomes.
It is not found in the Acrania and Cyclostomes, or any of the Invertebrates. It has been transmitted from the earliest fishes to all the Craniotes. The heart also, the central organ of circulation in all the Craniotes, shows an advance in structure in the Cyclostomes.
Opposed to the Acrania is the second division of the vertebrates, which comprises all the other members of the stem, from the fishes up to man. Hence they are called the Craniota. These Craniotes are, however, without a skull in their earlier period.
We can say, for instance, that we have inherited the oldest organs of the body, the external skin and the internal coat of the alimentary system, from the Gastraeads; the nervous and muscular systems from the Platodes; the vascular system, the body-cavity, and the blood from the Vermalia; the chorda and the branchial gut from the Prochordonia; the articulation of the body from the Acrania; the primitive skull and the higher sense-organs from the Cyclostomes; the limbs and jaws from the Selachii; the five-toed foot from the Amphibia; the palate from the Reptiles; the hairy coat, the mammary glands, and the external sexual organs from the Pro-mammals.
We may not say that "Amphioxus is the ancestor of the Vertebrates"; but we can say: "Amphioxus is the nearest relation to the ancestor of all the animals we know." Both belong to the same small family, or lowest class of the Vertebrates, that we call the Acrania. From this group of Acrania both the Amphioxus and the Craniota were evolved.
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.
They are much more advanced than the Acrania, much less so than the fishes, and thus form a very welcome connecting-link between the two groups. We may therefore consider them a special intermediate group, the fourteenth and fifteenth stages in the series of our ancestors. The few surviving species of the Cyclostoma are divided into two orders the Myxinoides and the Petromyzontes.
Word Of The Day