174 ARCHICG@LOMATA. 
Fig. 101.—ViEw oF Echinus Microstoma. 
(After WyviLtE THomson.) 
A, Internal View of the Skeleton showing Aristotle’s Lantern in position. 
B, Aristotle’s Lantern or Dental Pyramid. 
Crass IV.—Crinoidea. —The Crinoids have five jointed arms which 
bifurcate at the base, forming ten. Each has a number of pinne or 
small processes containing the gonads. The crinoids are fixed by 
a long stalk or axis to the sea-floor either throughout life or for the 
earlier part of. it. They are known as the stone-lilies and are 
mostly deep-sea forms. 
Fig. 102.,—ViIEW OF INTERIOR OF BISECTED SEA-URCHIN 
(Echinus Lividus ). 
Note the long coiled intestine suspended to the body-wall by mesentery. 
@, Gullet. a, Anus. 
2, First coil of intestine. ca, Ocular plate. 
m, A jaw-muscle. z, Intestine. 
“po, Cut end of a radial vessel. Z, Second coil of intestine. 
s, Part of the dental pyramid. ?, Radial water-vascular vessel. 
v, Ovary. 
Crass V.—Holothuroidea (the Sea-Cucumbers).—These have the body 
elongated in an oral-aboral direction, in some cases simulating a 
‘¢worm.” The ambulacra run in five rows down the sides of the 
body and, in addition, there is a ring of branching tentacles round 
the mouth. They have scattered calcareous spicules in the body- 
wall which give it a tough but flexible consistency. 
