ECHINODERMATA 
HE echinoderms, although their shapes are very unlike in 
the different classes, have the same general internal struc- 
ture, and also other features which place them together in one 
group. They are radiates (page 113) of the highest type; they 
have an exoskeleton; and many of them are beset with spines, 
from which the name is given. They have locomotor organs, the 
ambulacra, and a water-vascular system peculiar to themselves. 
Some have the very strange power of casting off, and developing 
again, parts of the body. 
The development from the egg to the adult is remarkable, and 
of unusual interest to naturalists. The larval stage of echino- 
derms is so unlike the mature animal that for a long time the 
larvee were thought to be another class of animals, and therefore 
were given the names which they still retain. 
In observing echinoderms it will be seen that the dorsal part 
is carried uppermost by some, such as the starfishes and sea- 
urchins. In the former the back is broad and extended; in the 
latter it is curved and contracted. But in crinoids the back is 
carried downward and is extended like a stalk; in the sea-cucum- 
bers (holothurians) the dorsal and ventral surfaces are parallel 
with the long axis of the cylindrical body, instead of being on a 
plane with the mouth and excretory opening. Owing to the 
unusual positions of the dorsal and ventral surfaces, the terms oral 
(mouth side) and aboral (side opposite the mouth) are generally 
used in describing these species. 
The surface of the body is divided definitely by the ambulacra, 
the ambulacral zones, and the interambulacral spaces, but differ- 
ently in the different classes. The movement of the animals is 
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