Embryology - 25 Apr 2024 Expand to Translate
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Gladstone RJ. and Wakeley C. The Pineal Organ. (1940) Bailliere, Tindall & Cox, London. PDF

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

The Pineal Organ - Eyes of Invertebrates

Chapter 4 Coelenterates

The simplest type of sense-organ which has been proved experimentally to be sensitive to light is represented by the eye-spots or " ocelli " situated between the bases of some of the tentacles in certain forms of Medusae, e.g. the jelly-fish, Aurelia aurita (Fig. 52, A). They consist of specialized groups of ectoderm cells containing red or black pigment, which are sometimes phosphorescent, or luminous in the dark, and are arranged round the margin of the umbrella. They are found on the outer side of a hollow, club-shaped process called the tentaculo-cyst (Fig. 52, B, C), which lies between each pair of marginal lappets. These marginal lappets, of which there are eight, are situated between the tentacles around the edge of the umbrella (Fig. 52, A, B). They lie in relation with the outer end of each " radial " and " per-radial " canal and between each pair is a prolongation of the " circular " canal, namely the tentaculo-cyst.

Fig. 52. — Diagram showing the Position of the Eye-spots of a Jelly Fish, Aurelia aurita. (After Lankester, from A Textbook of Zoology, Parker and Haswell.)

A — Side view. A segment of the umbrella has been cut away to show the interior. B — A small part of the edge of the umbrella to show the position of the tentaculo cyst and marginal lappets. C — Vertical meridional section showing the position of the eye-spot.

Fig. 53. — Diagram showing the Origin of the Central Nervous System and Primary Nerve Trunks from a Sub-epidermal Nerve Net.

A — Low type of flat worm ; B — Higher type (Planaria) ; C — Arthropod.

(After Jijima Hatschek, and Stempel.)

Another type of sense-organ which must be considered along with the ocelli is the " statocyst." These frequently contain calcareous particles which are supported on cilia and constitute one of the earliest types of equilibrating organs, such as are represented in vertebrates by the paired systems of semicircular canals and vestibules of the internal ear. Such equilibrating organs are found in Hormiphora plumosa, one of the combbearing free-swimming coelenterates. The class Ctenophora to which this species belongs is, moreover, of special interest in tracing the history of the sense-organs owing to the occurrence in these animals of a definite bilateral symmetry, as indicated by right and left tentacles and more especially, as in Hydroctena, by the presence of an ampulla at the apex containing two lithites supported on spring-like epithelial processes. Pigment spots are arranged as in the more primitive coelenterates circumferentially round the body. Moreover, the development of a definite sub-epithelial plexus of nerves in connection with the sense-organs beneath the general surface of the body and in the tentacles is worthy of note, as this sup-epithelial plexus of nerve-fibres and cells is the precursor of the central nervous system of the higher types of invertebrate animals (Fig. 53), and it is generally supposed that the central nervous system of the prevertebrate ancestors of the vertebrates arose in the same way.

circ. c. : circular canal.

gp. : gastric pouch.

ir. c. : interradial canal.

h. : hood.

/. .- lithite.

mg. I. : marginal lappet.

mth. : mouth.

oc. : eye-spot.

olf. \ olf.' 1 : olfactory pits.

or. a. : oral arms.

st. : stomach.

t. : tentacles.

tc. : tentaculo-cyst.

Historic Disclaimer - information about historic embryology pages
Pages where the terms "Historic" (textbooks, papers, people, recommendations) appear on this site, and sections within pages where this disclaimer appears, indicate that the content and scientific understanding are specific to the time of publication. This means that while some scientific descriptions are still accurate, the terminology and interpretation of the developmental mechanisms reflect the understanding at the time of original publication and those of the preceding periods, these terms, interpretations and recommendations may not reflect our current scientific understanding. (More? Embryology History \| Historic Embryology Papers)

Cite this page: Hill, M.A. (2024, April 25) Embryology Book - The Pineal Organ (1940) 4. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Book_-_The_Pineal_Organ_(1940)_4

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