CHAPTER XIX 



The Echinoclerms 



{Phylum Echmodcrmata) 



(Left) sea star, serpent star and sea cucumber; 

 ( right ) sea lilies and ( below ) sea urchin 



o 



NE of the delights in visiting the seashore is to 

 find sea stars (starfishes) and sea urchins, brittle 

 stars, and sand dollars. Or perhaps a sea cucumber. 

 Their symmetries, their strange movements, are fas- 

 cinatingly different from those of any creature found 

 on land. Even after a naturalist has enjoyed years of 

 acquaintance with echinoderms, they remain a great 

 enigma. Almost none of their actions resembles the 

 activities of other kinds of animals. Yet in their em- 

 bryonic development and some features of the adult 

 animal, they show remarkable similarities to chor- 

 dates such as ourselves. 



The name echinoderm comes from the Greek 

 echinos, a hedgehog, and derma, the skin. The word 

 is most suited to sea urchins, whose bodies are armed 

 with movable spines. 



A sea urchin or sand dollar differs from a sea star 

 or brittle star in that its skeleton is composed of inter- 

 locking plates that cannot be moved. The stars, by 

 contrast, can be real contortionists if given time to 

 change position. When first picked up a star may 

 seem stiff. But its skeleton, just inside the skin, con- 

 sists of separate pieces, each hinged movably to 

 neighboring ones. Muscles keep the star from feeling 

 flexible in human hands. Sea cucumbers may have 

 granules of lime embedded in the skin, but the body 

 wall is comparatively soft. 



A century and a half ago, the great French zoolo- 

 gist Baron Georges Cuvier grouped the echinoderms 



with the jellyfishes as "radiate" animals. But when the 

 development of echinoderm eggs is followed, each 

 embryo is found to develop into a bilaterally sym- 

 metrical larva. Later it takes on a modified radial 

 symmetry with five similar sectors. Since the animal 

 develops no head end, it comes to show distinctly only 

 an oral surface bearing the mouth and an aboral 

 surface opposite this. 



Sea cucumbers are unique among echinoderms in 

 giving up the radial pattern after acquiring it. They 

 lie over on one side, and thereby gain anew a distinc- 

 tion between right and left, between upper and lower 

 surfaces. 



Sea lilies are found almost exclusively at depths 

 that neither a beachcomber nor a skin diver can ex- 

 plore. They live permanently attached by long, slen- 

 der stalks to the bottom, and are so fragile that their 

 remains are unrecognizable when washed ashore. 

 Their more modern relatives, the feather stars, in- 

 habit also waters nearer land. They begin a sedentary 

 life, but become detached and can swim gently by 

 convulsive flapping of the arms. 



In all of these animals the body cavity is sub- 

 divided, one portion forming a water-vascular system 

 peculiar to echinoderms. This system consists of a 

 ring-shaped tube encircling the gullet, and five radial 

 tubes ("canals") extending into the five sectors of 

 the body. This hydraulic system receives its fluid 

 either from the body cavity (in sea lilies, feather 



254] 



