22 GENERAL PRINCIPLES OF ZOOLOGY. 



termed them, the zoophytes, to those animals with radially 

 symmetrical structure (Echinoderms and the floiver-ani- 

 mals) ; separating all the others, he formed of the unicellular 

 organisms standing lower the branch of " primitive animals" 

 or Protozoa; the higher organized animals he grouped 

 together as worms or Venues; at the same time he trans- 

 ferred a part of the Articnlata, the annelids, to the worm 

 group, and proposed for the other articulates, crabs, milli- 

 pedes, spiders, and insects, the term Artliropoda. 



Leuckart a decade later separated the branch Radiata 

 into two branches of very different degrees of organization. 

 The lower forms, in which as yet no special body-cavity is 

 present, the interior of the body consisting only of a sys- 

 tem of cavities serving for digestion, the alimentary canal, 

 he called the Ccelentera (essentially the Zoophyta of the older 

 zoologists); to the rest, in which the alimentary canal and 

 the body-cavity occur as two separate cavities, close beside 

 each other, he gave the name Echinoderma. 



The Present System. Thus there resulted seven 

 classes : Protozoa, Ccelentera, Echinoderma, Vermes, Ar- 

 thrppoda, Mollusca, and Vertebrata. Still this arrange- 

 ment does not correspond to the views which we are 

 justified in attaching to a natural system, and is hence 

 more or less unsatisfactory. Upon the ground of impor- 

 tant anatomical and embryological characters the BracJii- 

 opoda, the Bryozoa, and the Tunicata have been separated 

 from the Mollusca ; they form the subject of diverse opin- 

 ions. The conditions of relationship of the first two 

 groups have not yet been explained : of the Tunicata we 

 know indeed that they are related to the Vertebrata, yet 

 we cannot range them with these, since they show such 

 quite essential differences in structure. Of late there has 

 been a noticeable tendency to raise these small aberrant 

 groups to independent branches of the animal kingdom, 

 a procedure which can only lead to an injury to the com- 

 prehensibility and the practical value of the system. It 

 seems therefore preferable to regard these forms as ap- 

 pendages of the branch of worms, and in this book the 



