THE CLASSIFICATION OF ANIMALS 49 



teristics. Birds seem isolated, but they are closely con- 

 nected with reptiles by fossil forms. Even the great 

 gap in the animal kingdom that separating verte- 

 brates and invertebrates is partially bridged on the 

 one side by amphioxus, and on the other by balano- 

 glossus (a wormlike animal) and the tunicates. 



We have, then, groups subordinate to groups, and 

 interlocking, but not representing so many successive 

 degrees of organization. For, as already intimated, 

 complication of structure does not rise in continuous 

 gradation from one group to another. Every type starts 

 at a lower point than that at which the preceding class 

 closes ; so that the lines overlap. While one class, as a 

 whole, is higher than another, some members of the 

 higher class may be inferior to some members of the 

 lower one. Thus, certain starfishes are structurally 

 more complex than certain mollusks ; and the nautilus 

 is above the worm. The groups coalesce by their in- 

 ferior or less specialized members; e.g., the fishes do not 

 graduate into amphibians through their highest forms, 

 but the two come closest together low down in the 

 scale. Among the craniate animals the lines of descent 

 of the various classes may be represented as diverging 

 and ascending from a point occupied by a fishlike 

 ancestor. 



A number of animals may, therefore, have the same 

 grade of development, but conform to entirely different 

 types. While a fundamental unity underlies the whole 

 animal kingdom, suggesting a common starting point, 

 we recognize several distinct plans of structure. 5 Ani- 

 mals like the amoeba, with no cellular tissues and no 

 true eggs, form the branch Protozoa. Animals like the 

 sponge, with independent cells, one excurrent and many 

 incurrent openings, form the branch Porifera. Animals 

 like the coral, unlike all others, have an alimentary canal 

 DODGE'S GEN. ZOOL. 4 



