THE PHYLA HEMICHORDATA AND ECHINODERMATA 



365 



(Fig. 19.6), is at times abundant and may seriously deplete whole popula- 

 tions of oysters. Because ot its economic importance this starfish has been 

 studied extensively. 



Its color is variable, including shades of brown, yellow, orange, pink 

 and purple. The five arms or rays are joined at the center to form a 

 disc. On its upper surface the disc bears a bright orange or yellow 

 madreporite, a fine-meshed sieve that opens into a part of the coelom. 

 The eccentric location of the madreporite is the only obvious departure 

 from radial symmetry in the starfish. 



Asterias is protected from predators by a spiny skeleton in the meso- 

 derm just beneath the epidermis. A layer of calcareous plates (Fig. 

 19.6 D) comiected by short bands of connective tissue and muscle forms 

 a tough barrier. In addition many of the plates bear tubercles and 

 spines. The former are mere bumps whereas the latter are jointed at 

 the base and supplied with muscles so that they can be pointed in vari- 

 ous directions. Spines bordering the ambulacral grooves are especially 

 long and numerous, and can be closed over the grooves to protect them 

 if the starfish is torn loose from the bottom. Each skeletal piece is se- 

 creted as a single crystal of calcium carbonate. Although all of the 

 skeleton is originally covered with epidermis, that on the spines is 

 often worn off. 



The settling mud and the larvae of various organisms seeking places 

 to attach are threats to a slowly moving creature. In echinoderms the 



Ambulacrsd 

 plate 



Ampulla 



Tube 



Madreporite' 

 Anus 

 Rectal sac — ' 



-"Dermalgill 



Spines 



Figure 19.6. Asterias viewed from above with the arms in various stages of dissec- 

 tion. A, Arm turned to show lower side. B, Upper body wall removed. C, Upper body 

 wall and digestive glands removed, with a magnified detail of the ampullae and ambu- 

 lacral plates. D, All internal organs removed except the retractor muscles, showing the 

 inner surface of the lower body wall, E, Upper surface, with a magnified detail showing 

 surface features. 



