from The Century Dictionary.
- noun [NL., fem. sing.] In teratology, a malformation consisting in an entire absence of the bones and integuments forming the vault of the skull. Also written
- noun [capitalized] [NL., neut. pl.] A name proposed by Haeckel as a class designation for Amphioxus or Branchiostoma; a synonym of Myelozoa or Leptocardia (which see). Also called
Acephala. See Amphioxusand Branchiostoma.
from the GNU version of the Collaborative International Dictionary of English.
- noun (Physiol.) Partial or total absence of the skull.
- noun (Zoöl.) The lowest group of Vertebrata, including the amphioxus, in which no skull exists.
from Wiktionary, Creative Commons Attribution/Share-Alike License.
- noun physiology Partial or total absence of the
Sorry, no etymologies found.
At the ninth stage, called the skull-less animals (acrania), and corresponding to the still living lancelet, we enter the series of the vertebrates.
The brain and nervous system display the same progressive ascent from the brainless acrania, up through the fishes, batrachia, reptiles, and birds to the top in mammals.
The second chief section of the vertebrate-gut, the trunk or liver-gut, which accomplishes digestion, is of very simple construction in the acrania.
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. Chapter 2.21).
One has only to compare carefully the illustrations given (Figures 1.85 to 1.91) to see that, as a fact, the cenogenetic coelomation of the amphibia can be deduced directly from the palingenetic form of the acrania (Figures 1.79 to 1.84).
The gill-clefts, which originally in the older acrania pierced the wall of the fore-gut, and the gill-arches that separated them, were presumably also segmental, and distributed among the various metamera of the chain, like the gonads in the after-gut and the nephridia.
In other higher vertebrates, also, the kidneys develop (though very differently formed later on) from similar structures, which have been secondarily derived from the segmental pronephridia of the acrania.
It is also very pronounced in the young larvae of the cyclostoma; this fact is particularly interesting, as this palingenetic larva-form is in other respects also an important connecting-link between the higher vertebrates and the acrania.
In this case, again, it is clear that we must regard the features of the younger craniota as cenogenetically modified processes that can be traced palingenetically to the older acrania.
Amphioxus, the sole surviving representative of the acrania, once more yields us most interesting information; in this case the sexual glands remain segmented throughout life.