32 



Ecninoaerms — Forerunners oi tn( 

 Vertenrates 



Their Relatives. The starfishes and their relatives are animals whose body 

 plan, except in the developing young ones, is utterly different from that of any 

 other animals. A clue to their possible kinship comes from the resemblance 

 of the larvae to those of certain primitive chordates clearly related to the 

 vertebrates. Because of this the echinoderms have been promoted, by general 

 but by no means unanimous opinion, to a position near the chordates. 

 Although mollusks and insects have reached high peaks of invertebrate spe- 

 cialization and would seem to belong in that place, their larvae resemble those 

 of annelid worms more than those of any chordate. And young animals are 

 tell-tales of the origins of their parents. 



Characteristics. As the name of the Phylum Echinodermata, spiny-skinned, 

 implies, many of these animals are armed with hard, chalky and in some species 

 very heavy spines. Except in the larvae, they are radially symmetrical on a 

 plan of five or multiples of five that is unique (Figs. 32.1, 32.2). Even in 

 adults, however, they show signs of bilateral symmetry such as the position of 

 the sieve plate in the starfish through which a line may be drawn separating the 

 body into right and left halves. The bilateral symmetry of the free-swimming 

 larva (Fig. 32.8) is generally regarded as the fundamental plan upon which 

 the radial one has been overlaid during a long evolution. 



All echinoderms live in salt water and are thoroughly and curiously adjusted 

 to this existence. Their bodies are continually drenched with sea water, inside 

 and out. Their blood is practically sea water. A starfish cannot take one step 

 unless its watery blood flows into its foot. Oxygen diffuses into the blood and 

 carbon dioxide diffuses out of it through the thin walls of hundreds of skin-gills. 



There are no special excretory organs but slowly circulating fluid is con- 

 tinually washing the tissues and carrying away their by-products. Cilia are 



651 



