I30 ECHINODERMATA 



by numerous cilia. These food grooves may be on outer 

 surface and are rarely extended into free branches, the arms. 

 Anal opening excentric, often closed by a valvular pyramid. 

 Cystoids are the oldest and least specialized group of 

 Pelmatozoa (which include cystoids, blastoids and crinoids). 

 Class 7. Blastoidea (Gr. blastos^ a bud; eidos^ form) — extinct. 

 Confined to the Paleozoic, ranging from the Ordovician to the 

 Permian. Calyx ovate, short stemmed or stemless; distinct 

 arms absent, existing only as pinnules. Ten spiracles around 

 mouth, connected internally with hydrospires. Some have 

 a distinct anal opening; in others this is fused with one of the 

 spiracles. From the mouth radiate five ambulacra! areas. 



Characteristics 



I. Radially symmetrical; larvae bilaterally symmetrical. 



1. Calcareous skeleton, sometimes in plates which fit into each other 



to form a shell; sometimes in the form of scattered particles 



or spicules. 



3. In many the surface is beset with spines or tubercles. 



4. Never move rapidly in an adult condition; some are fixed by a 



stalk. 



5. Never bud to form a colony. 



6. All marine. 



7. Water vascular system (coelomic in origin) used for locomotion 



and to open bivalve molluscs. 



8. Body cavity well developed in the disc and usually in the arms, 



and separate from the digestive cavity. 



Natural History 



Class 1. Asteroidea. Type of Group — Asterias forbesii. 

 Starfish. (Figure 55.) — The starfishes have a star-shaped body, 

 with a central disc and five radiating arms, each of which contains a 

 prolongation of the body cavity and the organs belonging to the 

 digestive, reproductive and water-vascular systems. On the dorsal 

 side one finds the anus and the madreporic plate, while on the ventral 

 surface the tube feet protrude from five narrow grooves. 



External Anatomy. — The upper (aboral) surface is distinguished 

 by the presence of many spines of various sizes; pedicellariae at the 

 bases of the spines; a madreporite which is the entrance to a water- 

 vascular system; and anus. 



