chap, xv Backboneless Animals 235 



13th Class — Eolyzoa or Bryozoa, with one exception forming 

 colonies by budding, in fresh water or in the sea, e.g. the 

 common sea-mats or horn-wracks (Flustra). 

 14th Class — Brachiopoda or Lamp-shells, a class of marine 

 shelled animals once much richer in members, now 

 decadent. They have a superficial, but only a superficial, 

 resemblance to Molluscs. 

 I have not catalogued all these classes of " worms " without a 

 purpose. To ignore their diversity would have lent a false simplicity 

 to our survey. If you gain only this idea that there is a great 

 variety — a mob — of worm-like animals, which zoologists have not yet 

 reduced to order, you have gained a true idea. The " worms " lie 

 as it were in a central pool among backboneless animals, from which 

 have flowed, many streams of progressive life. They have affinities 

 with -Echinoderms, 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. 



4. Echinodermata. — In contrast to the " Worms," the series 

 including starfishes, brittle - stars, feather - stars, sea-urchins, and 

 sea-cucumbers, is well defined. 



The Echinodermata 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. More- 

 over, the larvse 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 worm-like animals are much simpler, much nearer the 

 hypothetical gastrula-like ancestor than are any Echinoderms, and 

 also because it is likely that Echinoderms originated from some 

 worm type or other. 



Haeckel used to hold a theory of the starfish which was in some 

 ways suggestive. You know the five-rayed appearance of the 

 animal like a conventional star ; you have perhaps watched it 

 moving slowly in a deep rock pool by the shore ;. you have 

 perhaps discovered that it will surrender one of its arms when you 

 try to capture it. Now Haeckel compared the starfish to a colony 

 of five worms united in the centre. Each "arm" or "ray" is 

 complete in itself. Each has a nerve -cord along the ventral 

 surface, a little eye at the .tip, prolongations of the food-canal, blood- 

 vessels, and reproductive organs. Each is anatomically comparable 



