236 The Study of Animal Life part in 



to a worm. Furthermore, when an arm is separated, it may bud 

 out other four arms and thus recreate an entire starfish. Each arm 

 has therefore some physiological independence. 



But there is no likelihood that a starfish arose as a colony of 

 worms ; the facts of development do not corroborate the sug- 

 gestion. 



Of Echinoderms there are seven classes, two of which are 

 wholly extinct. These — the Cystoids and Blastoids — are of great 

 interest because of their relationship with the feather -stars or 

 Crinoids, which stand somewhat apart from the other four extant 

 classes. The Cystoids are more primitive than the Crinoids, and 

 connect them with the starfishes or Asteroids. The Asteroids are 

 nearly related to the brittle-stars or Ophiuroids, and they are also 

 linked to the sea-urchins or Echinoids. These in turn are the 

 nearest allies of the Holothuroids or sea-cucumbers. 



The Echinoderms are all marine. The sea-urchins and Holo- 

 thurians are mud-cleansing scavengers ; the Holothurians and 

 Crinoids feed for the most part on small organisms, though the 

 former are sometimes mud-eaters ; the starfishes are more emphatic- 

 ally carnivorous, and often engnlf small molluscs. 



Among starfishes, sea-urchins, and sea- cucumbers, we find 

 occasional cases of prolonged external connection between the 

 mothers and the young. 



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), diflfer 

 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 pro- 

 tected by calcareous plates and spines. The sea-cucumbers are the 

 most muscular and the least limy, indeed in some almost the only 

 calcareous parts are a few anchors and plates scattered in the skin. 



Another frequent characteristic is the radial symmetry, but we 

 remember that the larvae are bilateral. 



Very important is the development ot 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, 

 and it is likely that in some at least they also help in excretion. 



Another characteristic of the Echinoderms is the strangeness of 

 the larval forms. For not only are they very different from the 

 parents, and very remarkable in form, but in no case do they 

 grow directly into the adult. The development is "indirect," 

 the larva does not become the adult ; the foundations of the 



