xii BACKBONELESS ANIMALS 233 



4. " Worms." This title is one of convenience, without 

 strict justification. For there is no class of "worms," 

 but an assemblage of classes which have little in common. 

 " Worm ' is little more than a name for a shape, most 

 of the animals so called differing from anemones and 

 jellyfish in having head, tail, and sides. The simplest 

 worms were apparently the first many-celled animals 

 to move persistently head foremost, thus acquiring 

 distinct bilateral symmetry, and a definite nervous centre 

 or brain in that region which had most experience the 

 head. The mesoderm, or middle layer of cells, which is 

 incipient in. Ctenophores, increases in importance. In 

 our survey we are helped by the fact that many worms 

 consist of a series of rings or segments, while others are 

 all one piece or unsegmented. It is generally true that 

 the latter are in structure simpler and more primitive 

 than the former. In the ringed worms or Annelids a 

 body-cavity or coelom appears a mesoderm-lined cavity 

 between the gut and the body-wall. Among 'worms,' 1 

 therefore, the series of Ccelomate, as contrasted with 

 Coelenterate animals, makes its beginning. 



1st Set of Worms. Plathelminthes or Flat Worms.- 

 Class : Turbellaria or Planarians.- -These are small 

 worms, living in the sea or in fresh water, or occasionally 

 in damp earth, covered externally with cilia, very simple 

 in structure, usually feeding on minute animals. The 

 genus Planaria, common in fresh water ; green species 

 of Vortex and Convoluta, which owe their colour to minute 

 alga? living in intimate partnership or symbiosis with 

 them ; Microstoma, which by budding forms temporary 

 chains of eight or sixteen individuals as if suggesting how 

 a ringed worm might arise ; Gunda, with a hint of internal 

 segmentation ; and two parasitic genera Graffilla and 

 Anoplodium may be mentioned as representatives of 

 this class. Specimens may be obtained by collecting 

 the waterweeds from a pond or seaweeds from a shore- 

 pool, and the simplicity of some may be demonstrated 

 by observing that when they are cut in two each half 

 lives and grows. 



Class : Trematoda or Flukes. These are parasitic 



