140 ZOOLOGICAL PHILOSOPHY 



(2) Echinoids. Skin neither irritable, nor mobile ; an anus. 

 Clypeaster (cake-urchin). Galerites. 



Cassidites. Nucleolites. 



Spatangus (heart-urchin). Sea-urchin. 



Ananchytes. 



(3) Fistulides. Elongated body, skin irritable and mobile ; an anus. 

 Holothuria (sea-cucumber). 

 Sipunculus [Gephyrean. H. E.]. 



Remark. Sipunculus is an animal very similar to the worms, but 

 its recognised affinities with the holothurians have caused it to be 

 placed among the radiarians, although it has not the characters of 

 that group and must therefore be placed at the end of it. 



As a rule, in a thoroughly natural classification the first and last 

 genera of the classes are those in which the standard characters are 

 least pronounced. Since the lines of demarcation are artificial, the 

 genera which are close to these Unes display the characters of their 

 class less conspicuously than the others. 



WORMS. 

 (Class IV. of the Animal Kingdom.) 



Suboviparous animals with soft elongated bodies, no head, eyes, legs, 

 or bundles of setae ; destitute of a circulation, and having a complete, 

 intestinal canal, that is, with two openings. 



Mouth consisting of one or several suckers. 



Observations. 



The general shape of worms is quite different from that of radiarians, 

 and their mouth, which is always formed as a sucker, has no analogy 

 with that of polyps, where there is merely an aperture associated with 

 radiating tentacles or rotatory organs. 



The worms in general have an elongated body, very shghtly con- 

 tractile, although quite soft ; and as regards their intestine they are 

 no longer limited to a single aperture. 



In the fistulide radiarians nature has begun to abandon the radiating 

 structure and to give an elongated shape to the bodies of animals, 

 the only shape which could conduct towards the end she had in view. 



After having fashioned the worms, she will henceforth tend to 

 estabhsh a type that is symmetrical as regards parts in pairs. She 

 could not have attained this type except through the type of articu- 

 lations ; but in the somewhat ambiguous class of worms, she has 

 merely sketched out the rudiments of certain features of it. 



