THE MOLLUSCS. 155 
the Protozoa—the Zoophytes and the Worms have developed, 
as two diverging branches. We must now in turn look 
upon the varied and much-branching tribe of Worms as the 
common primary group, out of which (from perfectly distinct 
branches) arose the remaining tribes, the four higher phyla 
of the animal kingdom. (Compare the Pedigree, p. 133.) 
Let us now take a genealogical look at these four higher 
tribes of animals, and try whether we cannot make out the 
most important outlines of their pedigree. Even should 
this attempt prove defective and imperfect, we shall at all 
events have made a beginning, and paved the road for 
subsequent and more satisfactory attempts. 
It does not matter in what succession we take up the ex- 
amination of the four higher tribes. For these four phyla 
have no close relationship whatever among one another, but 
have grown out from entirely distinct branches of the group 
of Worms (p. 133). We may consider the tribe of Molluscs 
as the most imperfect and the lowest in point of morpho- 
logical development. We nowhere meet among them with 
the characteristic articulation or segmented formation of the 
body, which distinguishes even the Ring-worms, and which in 
the other three higher tribes—the Echinoderma, Articulata, 
and Vertebrata—is most essentially connected with the high 
development of their forms, their differentiation, and per- 
fection. The body in all Molluscs—in mussels, snails, ete— 
is a simple non-jointed sack, in the cavity of which lie 
the intestines. The nervous system consists not of a cord 
but of several distinct (generally three) pairs of knots 
loosely connected with one another. For these and many 
other anatomical reasons, I consider the tribe of Molluscs (in 
spite of the high physiological development of its most 
