MANUAL OF NATUKAL HISTOEY. 331 



regular, molluscoid ; suckers mostly in three 

 rows, on a circumscribed disk on inferior sur- 

 face ; animal when moving turns up the two 

 extremities. 



III. ORDER. — True-Echinoderms (Cirrho-Spinigrada). 



Body covered with closely-jointed, calcareous 

 plates, mostly armed with spines ; lobes obscure ; 

 arms none ; progression by joint action of suckers 

 and spines. 



I. SUB-ORDER. — Aiolopygians (Aiolopygia). 



Vent eccentric, sometimes marginal. 



1. Family. — Heart-Urchins (Spatangidse). Body 



irregular, cordate or ovate ; some with long 

 slender spines ; mouth transverse, sub-cen- 

 tral or lateral, surrounded in some by ten- 

 tacles ; vent terminal ; ambulacra not con- 

 tinuous ; teeth none ; ovaries four. 



2. Family. — Helmet-Urchins (Galeritidas). Body 



conoid or sub-oval, base flattened; mouth 

 inferior, usually central ; vent marginal ; am- 

 bulacra extend from middle of back to mouth, 

 sometimes interrupted at the margin ; jaws 

 rudimentary. 



3. Family. — Shield-Urchins (Scutellidae). Body 



mostly flat and depressed, seldom convex, 

 margin orbicular or oval, sometimes rayed or 

 pierced with holes ; mouth inferior, central ; 

 vent sub-central or marginal ; ambulacra 

 petaloid, forming arched series on the middle 



