ECHINODERMA TA 



245 



and the young develope, being kept in position by some of the 

 spines bending over them. 



Fig. 147. — Spatangus purpureus. 



The Echinoidea are divided into three subdivisions : 



(i.) Regulares. — Sphaewidal or flattened circular bodies. 

 Ambulacral and interamtulacral areas equal in 

 length. Central mouth and subcentral amis. Complex 

 masticatory apparatus — Aristotle's Lantern — present. 

 Echinus, Toxopneustes. 



(ii.) Clypeastroidea. — Shield-shaped, often flattened todies. 

 Central mouth, with Aristotle's Lantern. Very broad 

 ambulacra, with their dorsal ends forming a peta- 

 loid rosette round the apical plate ; small tube -feet. 

 Anus excentric. Clypeaster, Eotula. 



(iii.) Spatangoidea. — Irregular heart-shaped bodies. Mouth 

 and anus excentric. No Lantern of Aristotle. Ambu- 

 lacra petaloid, and the anterior one unlike the others. 

 Spatangus, Brissus. 



Class V. THE HOLOTHUROIDEA (Sea Cucumbers). 



Characteristics. — Bchinodermata with elongated bodies, usually 

 pentagonal in cross section. The integument is leathery. 



