174 



ARCHICCELOMA TA. 



Fig. loi. — View of Echinus Microstoma. 

 (After Wyville Thomson.) 



A, Internal View of the Skeleton showing^Aristotle's Lantern in position. 

 B, Aristotle's Lantern or Dental Pyramid. 



Class IV. — Crinoidea. — The Crinoids have five jointed arms which 

 bifurcate at the base, forming ten. Each has a number of pinnae 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. — View of Interior of Bisected Sea-Urchin 



(Echinus Lividus). 

 Note the long coiled intestine suspended to the body-wall by mesentery. 



■ X\ ^\ 



ce^ Gullet. a^ Anus. 



/, First coil of intestine. ca, Ocular plate. 



?;/, A jaw-muscle. z", Intestine. 



po. Cut end of a radial vessel. /, Second coil of intestine. 



s. Part of the dental pyramid. p, Radial water-vascular vessel. 



z/, Ovary. 



Class 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. 



