CHAP. XV Backboneless Animals 235 



13th Class — Polyzoa 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 larv« 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 



