FUNDAMENTAL RELATIONS OF ANIMALS 81 



of these organic nests. The embryology of Cephalopods has been 

 masterly illustrated by Kolliker.^"'' 



There is still much diversity of opinion among naturalists respect- 

 ing the limits of Articulata, some being inclined to separate the 

 Arthropoda and Worms as distinct branches, while others unite 

 them into one. I confess I cannot see the ground for a distinction. 

 The worm-like nattire of the larvLc of the majority of Arthropods 

 and the perfect homology of these larvas with the true Worms seem 

 to me to show beyond the possibility of a doubt that all these animals 

 are built upon one and the same plan and belong therefore to one 

 branch, which contains only three classes, if the principles laid down 

 in my second chapter are at all correct, namely, the Worms, Crusta- 

 cea, and Insects. As to the Protozoa, I have little confidence in the 

 views generally entertained respecting their nature. Having satisfied 

 myself that Colpoda and Paramecium are the brood of Planariae, and 

 Opalina that of Distoma, I see no reason, why the other Infusoria, 

 included in Ehrenbersr's division Enterodela,^^^ should not also be 

 the brood of the many louver Worms, the development of which has 

 thus far escaped our attention. Again, a comparison of the early 

 stages of development of the Entomostraca with Rotifera might be 

 sufficient to show, -^vhat Burmeister, Dana, and Leydig have proved 

 in another way, that Rotifera are genuine Crustacea, and not Worms. 

 The vegetable character of most of the Anentera has been satis- 

 factorily illustrated. I have not yet been able to arrive at a definite 

 restilt respecting the Rhizopods, though they may represent in the 

 type of Mollusks the stage of yolk segmentation of Gasteropods. From 

 these remarks it should be inferred that I do not consider the Pro- 

 tozoa as a distinct branch of the animal kingdom, nor the Infusoria 

 as a natural class. 



Taking^ the class of Worms, in the Avidest sense, it would thus em- 

 brace the Helminths, Turbellarias, and Annulata. The embryology 

 of these animals still requires careful study, notwithstanding the 

 many extensive investigations to which they have been submitted; 

 the intestinal Worms especially continue to baffle the zeal of natural- 

 ists, even now when the leading features of their development are 

 ascertained. The Nematoids undergo a very simple development, 



i«9 [Rudolf Albert von Kolliker, 1817-1905.] 



^•^ That Vorticellidae are Bryozoa has already been stated above. 



