ECHINODERMATA 245 
and the young develope, being kept in position by some of the 
spines bending over them. 
SSS S 
= — —— —S-= SSS SS 
SSSSSSS ONS \ 
—————S = Sf es 
— —S== ge 
<< SS Z 
Fic. 147.—Spatangus purpureus. 
The Echinoidea are divided into three subdivisions : 
Gi.) Regulares.—Sphacroidal or flattened circular bodies. 
Ambulacral and interambulacral areas equal in 
length. Central mouth and subcentral anus. Complex 
masticatory apparatus—Aristotles Lantern — present. 
Echinus, Toxopneustes. 
(1.) Clypeastroidea.—Shield-shaped, often flattened bodies. 
Central mouth, with Aristotles Lantern. Very broad 
ambulacra, with their dorsal ends forming a peta- 
lod rosette round the apical plate; small tube - feet. 
Anus excentric. Clypeaster, Rotula. 
Gu.) Spatangoidea.—/rreqular heart-shaped bodies. Mouth 
and anus excentric. No Lantern of Aristotle. Ambu- 
lacra petalord, and the anterior one unlike the others. 
Spatangus, Brissus. 
Ciass V. THE HOLOTHUROIDEA (Sea Cucumbers). 
CHARACTERISTICS.— Lehinodermata with elongated bodies, usually 
pentagonal in cross section. The integument is leathery, 
