S LADEN : STRUCTURE OF ASTEROIDEA. 
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furrows along the rays and no sucker feet, only a pair of small 
semi-retractile tentacles to each joint, which have little or nothing 
to do with locomotion ; that function being performed by the 
specially-adapted, slender, flexible arms themselves. The internal 
anatomy presents still more striking differences, for the stomach is 
a simple sac confined entirely to the disk, and no prolongations of 
it in the form of digestive organs or other viscera are extended into 
the ray ; the interior of the ray being occupied by a great num- 
ber of vertebra-like joints : the whole arm forming a truly in- 
dependent appendage to the disk, instead of being an actual lobe of 
the body as is the case in starfishes. 
With this introduction we may now proceed to compare the 
special structure of what is generally regarded as a typical Asteroid 
with a typical Ophiuroid. 
The diagram given on Plate XV., Fig. 1, represents a transverse 
cut through one of the rays of a starfish. The section of the 
ambulacral furrow may be recognised on the lower margin, and it 
is now seen that this furrow is bounded, or indeed formed, by a 
calcareous arch, (marked a). This, however, is not a portion of a 
solid continuous roof or groove, as might naturally be supposed 
from such a drawing, but simply represents the end view of a pair 
of thin lamelliform plates : the length of the ray being built up of 
a great number of such pairs of plates placed end to end the whole 
way along. These plates are known as the 1 ambulacral plates,* and 
between each neighbouring pair, a pair of the ambulacral sucker- 
feet take their passage, (Fig. 1, t). The manner in which these 
organs are worked is very interesting, and takes effect in the 
following way : — On the upper surface of the disk-portion of a 
starfish a small round body may be seen, intersected by a number of 
fissures and resembling in miniature a brain- or madrepore coral. 
Thiough this sea-water passes freely, and traverses a filtering 
tube until it reaches a vein that encircles the mouth ; branches 
are given off from this oral vessel opposite each ray and 
proceed along the median line of the furrow ; small transverse 
veins being given off from this main trunk between each pair of 
