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BLADEN : STRUCTURE OF ASTEROIDEA. 
those border-lands that stand between neighbouring classes, and 
which from the very nature of their being, reflect to us some traces 
of the structure of ancestral forms that have long since passed away, 
or even of architypal phases with the actual persons of which we 
are altogether unacquainted. 
It will not perhaps be unprofitable to describe briefly and in 
plain words for the benefit of those who may not be acquainted 
with this branch of Zoology, the general appearance and character 
of the two sets of animals of which we are about to treat. 
The Asteroidea or Starfishes possess a more or less depressed 
body, which may vary in outline in every conceivable degree be- 
tween a goniodiscoid or pentagonal form, and a deeply indented or 
truly stellate form. The shape is maintained by a more or less 
compact frame or meshwork of calcareous pieces, over which is 
stretched a coreaceous skin, and which, in the generality of cases, is 
beset with a number of projecting spinelets or prickles. The 
mouth is situated in the centre of the under surface, and a deep 
furrow proceeds along each of the radii or ' arms,' as they are 
familiarly called. When the animal is alive and in health there 
may be seen piotruding from either side of the median line of this 
furrow, a long continuous row of white pellucid sucker-feet, with 
which the starfish crawls along. This area of the ray has been 
named the ambulacrum or ambulacral area, and the tube feet, the 
1 ambulacral suckers.' On making a dissection of a starfish it will 
be found that the stomach not only occupies the central part of 
the animal, but also that a portion of it, as well as of the digestive 
organs and other viscera, is extended along the rays or arms. 
Comparing now the Brittle Stars or Ophiuroidea, (which were 
so named from the long serpent-like arms they possess), we find a 
small depressed discoid body to which five or more long, worm-like, 
segmented appendages or rays are attached ; the disk being closely 
tessellated with scale-like plates and the rays being covered with 
four longitudinal series of large symmetrical plates, of which the 
lateral ones bear spines. The mouth is situated in the centre of 
the under surface of the body- disk, but there are no ambulacral 
