THE PHYLUM ECHINODERMATA 



mouth and anus in shifted locations. The paired coelomic sacs of the larva 

 are transformed into specific aduh structures: one of the pouches estabhshes 

 an external connection, the future madreporite, and develops into the water- 

 vascular system; other pouches give rise to various parts of the spacious 

 perivisceral coelom of the adult. It is perhaps significant, in interpreting 

 the evolution of echinoderms, that the change from bilaterality to radial 

 symmetry in the starfish occurs during a temporarily attached phase of the 

 life cycle. 



Regeneration. Starfishes are often found in nature with one or more arms 

 smaller than the rest. The small arms are in the process of regeneration. 

 Under experimental conditions as many as four arms may be removed with- 

 out causing the death of the animal, and all four can eventuallv be replaced 

 by regeneration. When all five arms are removed, regeneration can still 

 occur if the individual is fed after the formation of the new arms has begun. 

 Although an isolated arm of Aslenas can survive for several weeks, it will 

 eventually die, since it cannot restore the disk and other arms. In some other 

 starfishes, however, even a single isolated arm has the capacity to regenerate 

 into a complete individual. Under rough or damaging treatment a starfish 

 may shed an injured or restrained arm by a process termed autotomy. 

 A break of this kind always occurs at the base of the arm, where the body 

 cavity is restricted as it passes from disk to arm. The resultant opening in 

 the side of the disk is immediately closed by the contraction of the adjacent 

 body-wall musculature, and regenerative changes then begin. 



Other Asteroidea. In all the members of the class Asteroidea, the body 

 is stellate, whether the arms are long and slender, as in Henncia, or short 

 and broad, as in Oreaster. In some species more than five arms are present, 

 as in Solaster. In the leather stars, Dermastenas, spines are lacking, and 

 the body is covered by a smooth skin concealing the underlying reduced 

 skeleton. There are few members of the class so modified that they are not 

 immediately recognizable as asteroids. 



The Class Ophiuroidea 



Members of this class, the so-called serpent stars, brittle stars, and basket 

 stars, are recognizable by the relatively large and conspicuous disk and the 

 slender, mobile arms (Fig. 16.7). In basket stars the arms branch repeatedly 

 and terminate in many small, flexible tendrils. In ophiuroids the organs of 

 the digestive tract do not extend into the arms; the volume of the skeleton 

 is relatively much greater than in the asteroids, and the arms are largely 

 occupied by jointed skeletal units sometimes called "vertebrae." Brittle stars 

 are so named because of the fragility of their arms, which lash about actively 

 and break off very easily. In spite of their peculiarities, the ophiuroids are 

 clearly echinoderms. The life cycle (Fig. 16.8) includes a bilateral, ciliated 



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