22 GENERAL PRINCIPLES OF ZOOLOGY. 



termed them, the zoophytes, to those animals with radially 

 symmetrical structure (Echinoderms and the flower-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 }^crmcs\ at the same time he trans- 

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

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

 pedes, spiders, and insects, the term Arthropoda. 



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 Coclcntcra (essentially the Zoopliyta 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 Eckinoderma. 



The Present System. Thus there resulted seven 

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

 thropoda, 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 Brachi- 

 opoda, the Bryosoa, 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 w r ith 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 



