Talk:Book - The Pineal Organ (1940) 4

Chapter 4 Eyes of Invertebrates 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

rc.c

Fig. 52. — Diagram showing the Position of the Eye-spots of a Jelly Fish,

Aurelia aurita.

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.

[Continued at foot of next page.

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THE PINEAL ORGAN

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.

BRAIN & SUPRAOESOPHAGEAL GANGLION

PAIRED •-^•-r* LATERAL NERVES

WW

SUB-EPIDERMALNERVE NET W MEDIAN NERVE-CORD

ABC

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

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.

(After Lankester, from A Textbook of Zoology, Parker and Haswell.)

EYES OF INVERTEBRATES 83

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.