ECfflNODERM ATA — BL ASTOIDS 1 5 7 



shell. The hard protecting calyx is composed of many thin, 

 flexible and more or less overlapping plates ; these are perforated 

 by pores, which probably had a respiratory function. From 

 the centrally placed mouth, which is covered by four plates, 

 radiate five curved, unbranched, covered food grooves. The 

 anus occurs between two of the food grooves roofed over by a low 

 pyramid of plates. 



This genus and the many other forms similar to it are often 

 placed together in a distinct class, the Edrioasteroidea, extend- 

 ing from the Cambrian to the Mississippian. 



1. Sketch surface view, noting calyx, mouth, anus, am- 

 bulacra. 



2. How, probably, did the animal procure its food ? 



CLASS B, BLASTOIDEA (SEA-BUDS) 



Calyx ovate, short-stemmed or stemless ; distinct arms ab- 

 sent ; these exist only as pinnules. Around the central mouth 

 are five or ten spiracles which are connected internally with the 

 hydrospires, — canals probably respiratory in function. Some 

 forms have a distinct anal opening ; in others this opening is 

 fused with one of the spiracles. From the mouth radiate five 

 ambulacral areas. 



The class is entirely extinct and confined to the Paleozoic, 

 ranging from the Ordovician to the Permian. 



Derivation of name. — Greek blastos; bud, + eidos (oid), 

 form, in allusion to the bud-shaped calyx of most species. 



Pentremites (Fig. 61). Mississippian. 



This differs in general from the starfish in its bud-shaped 

 body (calyx), which was anchored to the ground by a short, 

 jointed stem attached to its dorsal surface ; the mouth, or ven- 

 tral side, was thus uppermost. The calyx inclosed the digestive 

 and blood vascular systems. In the center of the upper surface 

 is a five-angled opening, the mouth, surrounded by five round 

 openings, the spiracles. From each angle of the mouth radiates 



