THE WORMS. 149 
nearest akin to the Primzval 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 Accelomi arose the second main 
division of the Worm tribe, the Worms with blood and 
body-cavity (Ccelomati): of these there are seven different 
classes. 
The Pedigree on p. 151 shows how the obscure phylogeny 
of the seven classes of Ccelomati 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 
Ceelomati will at some future time throw light upon their 
phylogenesis. 
The Round Worms (Nemathelminthes) which we mention 
as the first class of the Ccelomati, 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 Trichine, 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 (Chzetopoda). Nearly akin to them are the Snout- 
worms (Rhynchoccela), and the small microscopic Wheel- 
