34 EVOLUTION OF LIFE. 
mon stock, from which are derived the Mollusca and Ver 
tebrata. Supposing the Worms to be the origin of these 
four divisions of the animal kingdom, an interest is excited 
in them not exceeded by any other group. One must 
expect then to meet with great difficulties in making a 
satisfactory arrangement of these organisms, to meet with 
structures that foreshadow much more complete types, to 
find many similar forms as well as individuals so different 
as to make it nearly impossible to say whether they belong 
tothe Worms at all. The classification as shown in Tree 
III. is principally that of Prof. Haeckel. It is true that 
this has met with objections, but it must be remembered 
that no tree could be constructed which would satisfy every 
anatomist. And this appears to be nearly in harmony with 
the views of the most eminent naturalists living. Bearing 
in mind that the nature of the subject will always make a 
classification with sharply-defined limits impossible, and as 
Haeckel says that this attempt, like every other of its kind, 
is provisional, to be corrected or confirmed by the specialists 
of the future, we will examine it a little more closely, 
being obliged, however, through the limits of this essay, 
to give only a general account. The Scolecida, or Soft 
Worms, supposed to have descended from Infusoria, divide 
into two groups,—the Flat Worms (Platyelminthes) and 
the Round. Worms (Nematelminthes). The Round Worms 
include the horse-hair worms (Gordiacez), so called from 
the superstition prevalent among country-people that the 
horse-hairs are changed into these worms. We find among 
them the Trichina spiralis, so famous in late years as the 
cause of the Trichiniasis, a disease resembling typhoid fever 
and acute rheumatism. The Trichina (Fig. 25) lives in 
its immature condition in a sac. These sac-like bodies are 
found in the pig, the meat of which, if not sufficiently 
cooked when eaten, will cause this very fatal disease, as 
the sac being destroyed by the juices of the intestine, the 
