xii BACKBONELESS ANIMALS 243 



without aTpurpose. Our aim has been to suggest that 

 there is a great variety a mob of worm-like animals, 

 which zoologists have not yet reduced to order. The 

 ' worms ' lie as it were in a central pool among back- 

 boneless animals, from which have flowed many streams 



J 



of progressive life. They have affinities with Echino- 

 derms, with Insects, with Molluscs, with Vertebrates. 



To practical people the study of " worms ' has no 

 little interest. The work of earthworms is pre-eminently 

 important ; the sea- worms are often used as bait ; the 

 leech was once the physician's constant companion ; 

 numerous parasitic worms injure man, his domesticated 

 stock, and the crops of his fields. 



5. Echinoderma. In contrast to the " Worms," the 

 series including starfishes, brittle-stars, feather-stars, sea- 

 urchins, and sea-cucumbers, is well defined. 



The Echinoderma are often ranked next the stinging 

 animals, mainly because many of the adults have a 

 radiate symmetry, as jellyfishes and sea-anemones have. 

 But radiate symmetry is a superficial character, perhaps 

 originally due to a sedentary habit of life in which all 

 sides of the animal were equally affected. Moreover, 

 the larvae of Echinoderms are bilaterally symmetrical, 

 that is to say, they are divisible into halves along a 

 median plane. We place Echinoderms after and not 

 before " worms," because the simplest wcrm-like animals 

 are much nearer the hypothetical gastrula-like ancestor 

 than are any Echinoderms, and also because there is some 

 reason to believe that Echinoderms originated from some 

 worm type or other. 



Of Echinoderms there are many classes, five of which 

 are represented nowadays in our seas, namely, star- 

 fishes or Asteroids, brittle-stars or Ophiuroids, sea- 

 urchins or Echinoids, sea-cucumbers or Holothuroids, 

 and feather-stars or Crinoids. Among the extinct classes, 

 such as Cystoids and Blastoids, there are some very 

 interesting types which show how the classes that have 

 persisted may have been linked together in the past. 



Many starfishes are very carnivorous, but others 

 depend a good deal on the lines of ciliated cells on their 



