IIo INVERTEBRATE ANIMALS. 
system, which are further protected by a sort of internal skele- 
ton. ‘The upper surface of the body and arms is richly furnished 
with calcareous matter, in the form of prickles, tubercles, spines, 
and pedicellariz, these last being peculiarly modified spines. 
The upper surface, also, exhibits the madreporiform tubercle 
in the form of a concentrically striated disc placed at the angle 
between two of the rays, and also the aperture of the anus, 
when this is present. The mouth is placed in the centre of the 
lower surface, and is not furnished with teeth. It leads by a 
short gullet into a stomach which usually terminates on the 
upper surface_by an anal aperture; but this is occasionally 
wanting. From the stomach in all the Asteroidea proceeds a 
series of much-branched membranous sacs, two of which are 
prolonged into each ray. The water-vascular or ambulacral 
system is in most essential respects identical in structure with 
that of the sea-urchins, making due allowance for the different 
shape of the body. The nervous system consists of a gangliated 
ring surrounding the mouth and sending branches along each 
of the arms. The reproductive organs, like the nervous sys- 
tem, exhibit a radiate condition, being arranged in pairs in 
each ray. 
The star-fishes are found on all our shores, but many forms 
are properly inhabitants of deep water. They differ much in 
the general shape of the body. In the common cross-fish 
(Uraster rubens) the disc is small, and is furnished with long 
finger-like rays, which are properly five in number. In the 
Cribella (fig. 44) the general shape is much the same. In the 
sun-stars (.So/aster) the disc is large and well marked, the rays 
are from twelve to fifteen in number, and they are shorter 
than the diameter of the disc. In the cushion-stars (Gozzaster) 
the body is in the form of a five-angled disc, more or less flat- 
tened on both sides, the rays being only marked out by the 
ambulacral grooves upon the lower surface. 
ORDER III. OpHIuROIDEA.—In this order we have only the 
common sand-stars (Ophzura) and brittle-stars (Ophzocoma), 
all closely allied to the true star-fishes in external appearance, 
especially in their strikingly radiate form. The body in the 
Ophiuride consists of a circular central disc covered with small 
calcareous plates, and giving off five long slender arms (fig. 45, 
a, 6), which may be simple or branched, but which do not con- 
