CHAPTER 



16 



SPINY-SKINNED 



The Phylum Echinodermata 



The Echinodermata are non-segmented, triploblastic forms showing a five- 

 part radial symmetry masking a fundamental bilaterality. Coelomic cavities 

 are extensive, forming in the embryo as outpocketings of the primitive diges- 

 tive tract. There is an endoskeleton composed of calcareous plates or spicules. 

 A unique characteristic of echinoderms is the presence of a system of fluid- 

 filled internal ducts, the so-called water-vascular system. In most echino- 

 derms the blood-vascular system is so reduced as to be non-functional and 

 very inconspicuous, and there are no traces of nephridial excretory organs. 



The phylum may be divided into the subphylum Pelmatozoa, containing 

 primitively stalked or attached forms, and the subphylum Eleutherozoa, 

 including free-moving, unattached echinoderms. The Pelmatozoa, once very 

 numerous, are now represented by only one modern class, the class Crinoidea, 

 sea lilies and feather stars. Several other pelmatozoan classes, among them 

 the Cystoidea and Blastoidea, contain extinct stalked forms known only as 

 fossils (Fig. 16.1). The Eleutherozoa contain the remaining four classes of 

 echinoderms: the class Asteroidea, sea stars or starfishes; the class Ophiuroidea, 

 brittle stars, serpent stars, and basket stars; the class Echinoidea, sea urchins 

 and sand dollars; and the class Holothuroidea, sea cucumbers. 



Echinoderms are typically slow-moving, bottom-dwelling animals. They 

 are exclusively marine, and there is no evidence from the fossil record that 

 members of the phylum have ever become established in fresh water. Because 

 of their radial symmetry, the echinoderms were at one time classed with the 

 coelenterates as "zoophytes." Further study, however, showed that 

 echinoderms are animals with an extensive coelom, which coelenterates lack, 

 and that even their radial symmetry differs markedly from the coelenterate 

 type. A significant feature of the life cycle of echinoderms is the occurrence 

 of a segmented, bilaterally symmetrical larva, which develops by a com- 

 plicated metamorphosis into the non-metameric, radially symmetrical adult. 

 The structure of the adult presents many puzzling characteristics when 

 compared with such familar types as mollusks, annelids, or chordates. If we 

 interpret the evolution of echinoderms from the events in the life cycle of a 



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