ASTEROIDEA. 255 
well-defined simple arms containing the gonads and prolonga- 
“tions of the gut, and with a ventral ambulacral groove 
supported by paired ossicles and bearing the tube-feet.; with 
regularly disposed calcareous, often spinous, plates on the skin ; 
with an external madreporite (occasionally multiple), always 
on the uppey surface of the disc in living forms; with a 
mouth at the centre of the lower surface, and usually with an 
anus at the opposite pole. 
Description of a Starfish. 
The description applies especially to the common five- 
rayed starfish (Asterias or Asteracanthion rubens). It is 
often seen in shore pools exposed at low water, but its 
haunts are on the floor of the sea at greater depths. There 
it moves about sluggishly by means of its tube-feet. 
Each of the five arms bears a deep ventral groove in 
which the tube-feet are lodged. The mouth is in the 
middle of the ventral surface, the food canal ends about 
the centre of the dorsal disc. With this flat, five-rayed 
form, the 11-13 rayed sun-star (So/as/er), the pincushion- 
like Porania, and the flat pentagonal Padmipes, should be 
contrasted. Between two of the arms lies the perforated 
madreporic plate, thus defining the d/vium, while the three 
other arms constitute the, ¢vivdum. | 
The body is covered by a ciliated ectoderm, beneath 
which lies a mesodermic layer. In association with the 
latter there is developed on the ventral surface of each arm 
a double series of sloping plates. These meet dorsally, like 
rafters, in the middle line of the arm, forming an elongated 
shed. The rafter-like plates are called ambulacral ossicles ; 
the groove which they bound lodges the nerve-cord, the 
water vessel, and the tube-feet of each arm. 
In association with the outer mesodermic layer of the 
integument, numerous smaller plates are developed, e.g. the 
adambulacrals, which articulate with the outer lower ends of 
ambulacrals. The dorsal surface bears a network of little 
ossicles, and many of these bear spines. Peculiarly modi- 
fied spines, known as pedicellariz, look like snapping 
scissor-blades mounted on a single soft handle. They 
