THE PHYLUM ECHINODERMATA 



Fig. 16.10. Generalized, sche- 

 matic life cycle of an echi- 

 noid echinoderm. Through 

 an indifferent pluteus stage 

 the gastrula (A) transforms 

 into a fully developed echi- 

 nopluteus (B); after a period 

 of larval life this meta- 

 morphoses into a young sea 

 urchin (C). (Adapted from 

 H. B. Fell, 1948, Biological 

 Reviews, vol. 23, printed by 

 permission of the Cambridge 

 University Press.) 



radial appendages of the digestive system as in the starfish. Aside from these 

 special features, however, the internal organization is similar to that of the 

 asteroids: the ambulacral and reproductive systems, as well as the nervous 

 system, are generally comparable in arrangement to those of the starfish, 

 although the circulatory system is better developed. 



In Arbacia the sexes are separate; the five gonads are radially arranged, 

 and each sheds its sexual products into the sea water through one of the pores 

 located on the genital plates. Fertilization is thus external, and development 

 proceeds rapidly to a bilateral, ciliated larval stage, the echinopluteus (Fig. 

 16.10). This larva is comparable to the bipinnaria of the starfish, although 

 in the presence of several pairs of long arms, each supported by an internal 

 spine, it resembles even more closely the ophiopluteus of the ophiuroids. 

 After several weeks of pelagic life, feeding on diatoms and microscopic algae, 

 the pluteus sinks to the bottom and undergoes metamorphosis into a miniature 

 sea urchin. 



Other Echinoidea. In Arbacia the body is circular in its lateral outline, 

 and the 20 rows of plates are arranged in a regular pattern. This regularity 

 of skeletal arrangement has not always been characteristic of sea urchins, as 

 shown by some fossil forms in which the plates are irregularly arranged. 

 Among the modern echinoids the shape and symmetry of the body are highly 

 variable (Fig. 16.11). In Clypeaster the mouth is in the center of the oral 

 surface, but the anus lies in an interambulacral area on the lateral margin 

 of the somewhat flattened body; thus the animal is actually bilaterally sym- 

 metrical. In the sand dollar, Echinarachnius parma, the organization is like 

 that of the clypeasters, although the flattening of the body is more extreme. 

 In the type represented by Spatangus, the mouth has shifted peripherally, or 



501 



