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POPULAR SCIENCE REVIEW. 
one continuous circular co mmi ssure lodged like the other cords 
in special canals. 
What is the meaning of this excessively complicated 
arrangement ? Experiment shows that the movements of the 
arms are dependent upon the integrity of their axial cords and 
upon the connection of these cords with the central fibrillar 
envelope of the chambered organ. The swimming movements 
of a Feather-Star are exceedingly active, and are also performed 
with a singular regularity. When a ten-armed animal swims, 
all the five right arms are simultaneously bent, and then the five 
left arms. As long as the swimming lasts this alternating 
movement is kept up with the most perfect regularity. Owing 
to the length of the arms and to the small size of their com- 
ponent joints, of which there are frequently more than one 
hundred to each arm, the number of muscles concerned in the 
movement reaches at least one thousand pairs. 
Experiment shows that these muscles are under the influence 
of a governing centre, which not only regulates their con- 
tractions, but co-ordinates these contractions in the most re- 
markable manner. This co-ordination is well shown in the 
following experiment. When one of the first pair of pinnules 
on the arm is irritated, the whole circlet of arms is suddenly 
and simultaneously closed over the disc ; hut irritation of one 
of the ordinary pinnules higher up the arm is simply followed 
by flexion of the arm which bears it. The governing centre on 
which this action depends has been shown to he situated in the 
fibrillar sheath of the chambered organ ; and the axial cords of 
the rays and arms are the channels by which the influence of 
the centre is Communicated to the muscles. For the swimming 
movements of the whole animal depend upon the integrity of 
the chambered organ, and may take place after the visceral disc 
has been removed from the skeleton, which contains the cham- 
bered organ and axial cords, and swims about on its own account. 
The swimming movements are therefore entirely independent of 
any structures contained in the disc. In the same way the 
movements of each individual arm depend upon the integrity 
of the axial cord of that arm, stopping directly it is injured ; 
and microscopic investigation shows that branches of the axial 
cord are distributed upon the ends of the muscular bundles 
which connect the successive joints of the rays, arms, and 
sometimes also of the pinnules. 
The above facts seem to show that the axial cords of the 
rays and arms, together with the fibrillar sheath of the chambered 
organ in which they originate, constitute a system of motor 
nerves of no little complexity. The only difficulty in the way of 
this view is that nothing of the kind is known in the other 
Echinoderms. The Star-fishes, for example, have a fibrillar 
band between the radial blood-vessel and the cellular lining of 
