THE WORMS. 149 



nearest akin to the Primaeval Worms. The parasitical 

 Sucker- worms (Trematoda) arose out of the Gliding-worms, 

 which live freely in water, by adaptation to a parasitical 

 mode of life ; and out of them later on — by an increasing- 

 parasitism — arose the Tape- worms (Cestoda). 



Out of a branch of the Acoelomi arose the second main 

 division of the Worm tribe, the Worms with blood and 

 body-cavity (Coelomati) : of these there are seven different 

 classes. 



The Pedigree on p. 151 shows how the obscure phylogeny 

 of the seven classes of Coelomati may be supposed to stand. 

 We shall, however, mention these classes here quite briefly, 

 as their relationships and derivation are, at present, still 

 very complicated and obscure. More numerous and more 

 accurate investigations of the ontogeny of the different 

 Coelomati wiU at some future time throw light upon their 

 phylogenesis. 



The Roxmd Worms (Nemathelminthes) which we mention 

 as the first class of the Coelomati, and which are character- 

 ized by their cylindrical form, consist principally of para- 

 sitical Worms which live in the interior of other animals. 

 Of human parasites, the celebrated Trichinse, the Maw- 

 worms, Whip- worms, etc., for example, belong to them. The 

 Star-worms (Gephyrea) which live exclusively in the sea are 

 allied to round worms, and the comprehensive class of Ring- 

 worms (Annelida) are allied to the former. To the Ring- 

 worms, whose long body is composed of a number of seg- 

 ments, all alike in structure, belong the Leeches (Hirudinea), 

 Earth-worms (Lumbricina), and all the marine bristle-footed 

 Worms (Chsetopoda). Nearly akin to them are the Snout- 

 worms (Rhynchocoela), and the small microscopic Wheel- 



