. Chordate anatomy. Chordata; Anatomy, Comparative. PHARYNX/ ,1 ST GILL APERTURE HYPOBRANCHIAL MUSCLE HYPOPHYSIAL ^ DUCT ^=-^ (RESPIRATORY TUBE. NOTOCHORD I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 206.—Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possi- bility that it may once have served as a mouth (paleostoma). In Petromyzon the hypo- physis fails to open into the pharynx
. Chordate anatomy. Chordata; Anatomy, Comparative. PHARYNX/ ,1 ST GILL APERTURE HYPOBRANCHIAL MUSCLE HYPOPHYSIAL ^ DUCT ^=-^ (RESPIRATORY TUBE. NOTOCHORD I ST GILL APERTURE B. PETROMYZON. HYPOBRANCHIAL MUSCLE Fig. 206.—Diagrams of median longitudinal sections of the heads of Bdellostoma and Petromyzon, showing the relations of the hypophysial ducts in the two forms. In the former the hypophysial duct opens posteriorly into the pharynx, suggesting the possi- bility that it may once have served as a mouth (paleostoma). In Petromyzon the hypo- physis fails to open into the pharynx and is converted into a pipette-like organ into which the olfactory pits open. On the basis of this difference cyclostomes are divided into two sub-classes, Hyperotreta and Hyperoartia. mouth becomes the blastopore of chordate embryos. And the blastopore of chordates lies at the posterior end of the body and forms the neurenteric canal, which connects the neural tube with the enteron, while the chordate mouth develops at the anterior end of the enteron. Consequently, it seems indisputable that there have been at least two mouths in the history of vertebrates. While, however, all agree that the vertebrate mouth is not the primary animal mouth, and that at least two mouths have successively appeared, some rnorphologists believe that there have been at least three mouths, Delsman (1922), reviving an earlier suggestion of Kowalevsky (1877), claims that " in the ontogeny of vertebrates we see three successive mouths appear in the same succession as they appeared in phylogeny, viz., the blastopore (Urmund), the neuropore (the annelidan mouth), and finally. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Neal, Herbert V. (Herbert Vincent), 1869-1940; Rand, Herbert W. (Herbert Wilbur), 1872-1960. Philadelphi
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Keywords: ., bookcentury1900, bookpublisherphi, booksubjectanatomycomparative
