KADIATA. 



189 



a 



a 



by delicate mesenteries to the shell. These animals pos- 

 sess a heart with an aorta surrounding the gullet and in- 

 testine. The blood 



is aerated by ex- o 



posure to the ox- 

 ygen mixed with 

 the water which is 

 constantly circula- 

 ting over the vas- 

 cular mesenteries. 



The metamor- 

 phosis of Echinus is 

 very curious. The 

 embryo is a free 

 swimming minute 

 ciliated creature, 

 strangely like a 

 painter's easel, and 

 hence called a Pluteus. \_Pluteus, a penthouse.] This 

 passes through a strange cycle of changes. The diges- 

 tive canal appears in the middle of the frame, which 

 gradually disappears, the future Echinus is sketched in, 

 and a radially symmetric animal results, totally unlike its 

 predecessor. (Fig. 91.) 



Regular Echini, as the common Cidaris, are nearly 

 globular, and the oral and anal openings are opposite. 

 Irregular Echini, as the Clypeaster and Spatangus, are 

 flat, or discoid, with hair-like spines, and the rows of am- 

 bulacra form a five-rayed star on the back of the shell. 

 Spatangus has no dental apparatus. 



The order HOLOTHUROIDEA, embraces what are com- 



FIG. 90. Dentary Apparatus of Echinus, or Aristotle's 

 Lantern. The right-hand diagram shows three of the teeth 

 in position, a a. Cutting edges of the teeth, which are ex- 

 tremelyhard. b. Fibrous roots of the teeth, cc. Opposed 

 bony surfaces of the jaws, d d. Arched processes. The 

 left-hand diagram shows an isolated pyramid, e. Exter- 

 nal surface. Other letters as before. 



