THE MOLLUSCS. 155 



the Protozoa — the Zoophjrtes 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 foxu' 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, etc. — 

 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 



