ON THE LOCOMOTOR SYSTEM OE ECHINODERMATA. 
847 
lantern seems to us mainly subservient to the execution of these movements, and yet 
—so far as we could detect—they are utterly useless for any purpose other than 
that of assisting locomotion. 
(D.) Pedicellarice. —A good deal of speculation has been expended on the probable 
function of these organs. Prolonged observation lias satisfied us that they have a 
function which has not hitherto been suspected, viz., that of assisting locomotion. A 
full account of our observations on these structures, however, had better be reserved 
for the next section of this paper. 
3. Spatangus.— This animal crawls about somewhat slower than Echinus, keeping 
its very long spines partly erect to act as feelers. It does not appear able to climb 
perpendicular surfaces. When placed upon its back, it has more difficulty in righting 
itself than any of the Echinoderms that we have observed; for, on account of its 
having such flat poles and such short ambulacral feet, it is, when inverted, placed at 
even a greater disadvantage than is Echinus. Therefore, many specimens—especially 
large specimens—are never able, when inverted on a hard flat surface, to right them¬ 
selves at all; smaller specimens, however, are able to do so after an expenditure of 
much time and energy. Their method of doing so is quite different from that of 
Echinus. Indeed, looking to the shape of Spatangus and to the character of its 
pedicels, the method of righting adopted by Echinus would be here clearly impossible. 
This animal, therefore, rights itself entirely by the action of the only organs which are 
available for the purpose, viz., its long and mobile spines. The long spines are not 
very many in number; but as their strength and co-ordination is surprising, they 
enable the animal, by a series of pushings and proppings, eventually to turn itself 
completely over from one of its flattened surfaces to the other. In doing this it 
usually, but not invariably, turns over upon its broad end. When the long spines are 
removed, the animal, of course, is no longer able to right itself. 
4. ITolothurians. —Very little has to be said on these sluggish members of the 
Echinoclerm group. They crawl slowly, and indulge in prolonged periods of quiescence. 
They are, however, able to climb perpendicular surfaces. 
From this account of the natural movements exhibited by the several groups of 
Ecliinodermata here considered, it may be observed that we have presented to our 
view an interesting series of graduated modifications. At one end of this series we 
have Echinus and Spatangus with their rays all united into a box-like rigid shell. 
At the other end of the series we have the Brittle-stars with their muscular rays, 
highly mobile, and indeed snake-like in their well co-ordinated movements. Midway 
in the series we have the Sea-cucumber and common Star-fish, where the body is 
flexible and mobile, though not so much so as in the Brittle-stars. Now, the interesting 
point to observe is, that in correlation with this graduated difference in the function 
of the rays, we have a correspondingly graduated difference in the development of 
the ambulacral system. In Echinus and Spatangus this system is seen in its most 
