This historic book may have numerous typos, missing text, images,
or index. Purchasers can download a free scanned copy of the
original book (without typos) from the publisher. 1900. Not
illustrated. Excerpt: ... CHAPTER II THE CENTRAL NERVOUS SYSTEM OF
MEDUSAE. EXPERIMENTS ON SPONTANEITY AND COORDINATION i. Experiments
on Medusae or jelly-fish afford us an excellent opportunity for
analysing the conditions for spontaneity and coordination, and for
deciding whether or not these phenomena are dependent upon
ganglion-cells. The subumbrella of the Medusae has a very thin
layer of muscle-fibres which contract rhythmically. The contraction
diminishes the size of the swimming-bell, and forces the water out.
By means of the recoil the animal moves forward. In regard to the
nervous system, we must discriminate between two classes of
Medusae: first, the Hydromedusae (Hydroidea, Fig. 1), and, second,
the Acalephae, one representative of which (Aurelia aurita, Fig. 2)
is familiar to many laymen. The nervous system of the Hydromedusae
consists of a double nerve-ring along the margin of the umbrella
(d, Fig. 1). The upper nerve-ring forms a flat layer in the
ectoderm, and consists of thin fibres and ganglion-cells. The lower
nerve-ring has thicker fibres and more ganglion-cells, and is
connected with the upper ring by nerve-fibres. In addition to this
ring, which is called the central nervous system, there is also a
peripheral nervous system, a plexus, consisting of nerves and
scattered ganglion cells, spread out over the whole subumbrella
(J>, Fig. 1), between the epithelium and the muscle-layer. The
convex surface of the umbrella consists of a non-contractile,
gelatinous mass, and no nervous elements are to be found in it.
Acalephae (Fig. 2) have no continuous nerve-ring, but a row of
separate nerve-centres (S, Fig. 2) extends around the margin of the
umbrella, lying in the ectoderm, which covers the basis of the
marginal bodies (sense organs). The number of these centr...
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