ECHINODERMATA 
tN 
i) 
wt 
Ciass I. ASTEROIDEA (Starfishes). 
CHARACTERISTICS.—Lehinodermata whose body is flattened dorso- 
ventrally, and is produced into arms or rays, which are usually 
jive or more ir number. These arms are longitudinally 
grooved on the ventral surface, and the tube-feet lie in this 
groove. The madreporie plate is dorsal and interradial in 
position. The alimentary canal sends caecal diverticula into 
the arms. The generative organs are interradial in position 
at the base of the arms.  Pedicellariae usually present. 
Asterias rubens is one of the commonest of starfishes, and is 
constantly left stranded on our shores by the retreating tide. 
Its body consists of a central disk, from which five arms or 
radii project. The surface on which it habitually rests or 
moves, and on which the mouth opens, may be termed the 
ventral, the upper and more convex, where the anus is situated, 
may be called the dorsal. 
From the mouth five grooves radiate along the arms, these 
are the ambulacral grooves, and they lodge the tube-feet; between 
each two grooves, and consequently interradial in position, are 
five sets of oral spines, which project over the mouth and perhaps 
assist in feeding. If the tube-feet be removed from each ray, 
it will be seen that the ambulacral groove is formed of two rows 
of ambulacral plates, situated right and left of the middle line of 
the radius (Fig. 131). Each right plate is so placed as to form 
an angle, open ventrally, with the corresponding left plate, and 
between the adjacent plates of each side certain pores exist 
which give exit to the tube-feet. The groove is covered in by 
the integument, and lodges two radial canals, of these the most 
ventral is divided by a vertical septum, and is called, for 
reasons mentioned below, the “ peri-heemal” space. The dorsal 
canal is the radial trunk of the water-vascular system. At the 
outer end of the ambulacral plate a series of adambulacral 
ossicles are situated, and these support three rows of moveable 
spines. Those spines which are nearest to the centre of the 
disk form the oral spines mentioned above ; these are borne by 
the first adambulacral ossicles, one set on each side of an inter- 
radius. 
At the distal end of each arm the ambulacral plates end in 
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