4 GENERAL SURVEY OF THE ANIMAL KINGDOM. 



is a name for a mere shape which animals of very diverse 

 structure have assumed. They have begun to move head 

 foremost, and to acquire sides. In this "mob" we may, 

 however, recognise that the simpler worm-like forms are 

 unjointed or unringed. They are thus contrasted with 

 ringed or segmented "worms," such as the earthworm. 

 But this segmentation appears gradually. In our dis- 

 cussion we shall observe the following order : — 



Flat Worms ( Turbellaria. Planarians (free-living), 

 or \ Trematoda. Flukes (parasitic). 



Plathelminthes. ( Cestoda. Tapeworms (parasitic). 



Nemertea. Ribbon-worms (free-living). 

 Nematoda. Thread-worms (many parasites). 

 Some small classes. 



, Chtetopoda. Bristle-footed worms, e.g., earth- 

 Annelids \ worm and lobworm. 



or < Discophora or Hirudinea. Leeches (external 



Ringed Worms. / parasites). 



^ Some small sets. 



iSipunculoidea, e.g., Sipunculus. 

 Bryozoa or Polyzoa, e.g. , Flustra, the sea-mat. 

 Brachiopoda. Lamp-shells. 



In the mean time, however, be assured that there is no 

 class of worms, but think of the assemblage of classes as in 

 the centre of the invertebrate animals, with affinities on all 

 sides, — to Echinoderms, Arthropods, Molluscs, and Verte- 

 brates. They lie in a pool from which many streams flow. 



It is probable that the well-defined series of Echinoder- 

 MATA — star-fishes, sea-urchins, etc., — had their origin in some 

 worm-like type. There are seven classes, of which two are 

 wholly extinct. 



Holothuroidea. Sea-cucumbers. 

 Echinoidea. Sea-urchins. 



!Asteroidea. Star-fishes. 

 Ophiuroidea. Brittle-stars. 



( Crinoidea. Feather-stars. 



Very different is the Arthropod series, including Crus- 

 taceans, Insects, Spiders, etc., jointed animals like the 



