46 University Geological Survey of Kansas. 



ECHINOIDEA. 



The "sea-urchins," "sea-eggs," "sand-dollars," and " sea- 

 cookies," as they are variously called, are animals varying in 

 form from spheres to discs. They live in moderately shallow 

 water, especially near oyster beds, while some bury themselves 

 in the sand, and one kind is known to make holes for itself in 

 the solid rock. They always inhabit the sea. The parts that 

 are generally found fossil are the spines and the plates, which 

 are here explained. 



The body of the animal is encased in a kind of box made up 

 of calcareous plates, which are united at their edges. A more 

 or less long spine is attached to each of certain plates. The 

 spines project out from the body in all directions. They are 

 attached to the plates by a sort of ball-and-socket joint, and are 

 movable. There are two kinds of plates — those with, and those 

 without, spines. The latter are called the ambulacral plates, 

 or the plates through which the tube-feet project, while those 

 with spines are called the interambulacral plates. The am- 

 bulacral plates are arranged in five bands, or areas, between 

 which the interambulacral plates are arranged. Each band 

 generally extends from the top to the center of the bottom of 

 the animal. 



The mouth is located at the center of the bottom side. In 

 some sea-urchins the mouth is provided with five long teeth 

 and a complicated set of calcareous supports. The whole ar- 

 rangement is known as "Aristotle's lantern." On the top 

 side of the animal near the center there is a sieve-like plate, 

 which connects with a long membranous tube beneath. This 

 tube runs downward to the base of the inner cavity and con- 

 nects with another tube which forms a ring around the esoph- 

 agus. There are five branches to this latter tube, one extending 

 along the center of each ambulacral area. In the ambulacral 

 plates there are series of holes through which the numerous 

 branches of these tubes extend to the outside of the shell. 



These branches, or tube-feet as they are called, extend out- 



