CHAP, xii BACKBONELESS ANIMALS 245 



surface, which sweep particles into the mouth. The 

 common sea-urchins browse on acorn-shells and seaweed ; 

 the heart-urchins press sand into their widely open 

 mouths. Some of the Holothurians plunge one feathery 

 tentacle after another (see fig. 70) into the mud and then 

 into their mouth. To a large extent Echinoderms feed 

 on minute living creatures and on organic detritus. 



The Echinoderms are sluggish animals, though many 

 brittle-stars are lithe gymnasts, and though the commonest 

 Crinoids (Comatulids, such as the rosy feather-star, 

 Antedon rosacea) differ from their stalked relatives and 

 adolescent stages in being to some extent swimmers. 

 Perhaps the sluggishness is expressed in the abundance 

 of lime in the skin and other parts ; for, as the name 

 suggests, the Echinoderms are thorny-skinned, being 

 usually protected by calcareous plates and spines. The 

 sea-cucumbers are the most muscular and the least limy, 

 in some indeed almost the only calcareous parts are a few 

 anchors and plates scattered in the skin. 



Very important is the development of a peculiar system 

 of canals and suctorial " tube-feet " the water- vascular 

 system. By means of the tube-feet the starfishes and sea- 

 urchins move, in the others their chief use seems to be in 

 connection with respiration. 



Of great interest is the primitive nature of the nervous 

 system, which often remains very superficial, retaining 

 its connection with the skin. But besides that, there 

 is the striking rarity of nerve-centres or ganglia. There 

 is little evidence of them except in some brittle stars. 

 This is particularly interesting, because some Echinoderms 

 exhibit quite a complex behaviour, for instance when a 

 starfish does battle with a sea-urchin. Moreover, the 

 experiments made by Prof. Jennings show very clearly 

 that a starfish is able to profit by experience. It can 

 do this although it has no nerve-ganglia, only rows of 

 nerve-cells. In other words, there may be considerable 

 complexity of behaviour though the structure of the 

 nervous system remains very simple. This confirms what 

 was noted in regard to organic purposiveness in the 

 Protozoa. 



