The Classification of Animals. 187 



is feebly developed, and at most consists of chinks and minute 

 cavities in the mesoderm. They have a pair of cerebral 

 ganglia, but no ganglionated ventral cord. In the free-living 

 forms the body is often ciliated. It is only among the 

 Nemertea that a vascular system is present. The nephridia 

 are branched and complicated, and terminate in " ilame cells " 

 — i.e. a single cell with a single flagellum attached, not a wide 

 and multicellular funnel, as in the Annelids. 



II. Nematoda. 



The typical members of this group are the parasitic thread- 

 worms, of which the thread-worms of the horse and the 

 Trichina are examples. But allied to this group, and possibly 

 to be included within it, are the marine and pelagic Chaetognatha. 

 The most striking peculiarity — which, however, they share with 

 the Arthropoda — is the complete absence of cilia in the true 

 Nematoda. The body is unsegmented in the latter, and the 

 alimentary canal lies in a spacious cavity which has not all 

 the characters of a true coelom. The members of this group 

 have often complicated life histories (as have also various 

 parasitic Platyhelminthes), passing part of their existence as 

 free-living worms and part shut up within the bodies of one 

 or more hosts. 



III. ECHINODERMA. 



This group of animals includes the starfishes, sea-urchins, 

 sea-lilies and fossil encrinites, sand-stars, and sea-cucumbers, or 

 holothurians. All the members of the group present a more 

 or less pronounced radial symmetry. In all there is a con- 

 siderable, often massive, calcareous skeleton, mesodermic, and 

 regularly arranged in plates and spines. The coelom is 

 spacious, and a part of the ccelom, in the shape of regularly 

 arranged canals, is known as the water vascular system. 



