THE CLASSIFICATION OF ANIMALS. 



233 



and Invertebrates — is partially bridged on the one side 

 by Amphioxus, and on the other by the Tunicates. 



We have, then, groups subordinate to groups, and inter- 

 locking, 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 infe- 

 rior to some members of the lower one. Thus, certain 

 Star-fishes are nobler than certain Mollusks ; the Nautilus 

 is above the Worm, and the Bee is more worthy than the 

 lowest Fish. The groups coalesce by their inferior or less 

 specialized members ; e. the Fishes do not graduate into 

 Amphibians through their highest forms, but the two come 

 closest together low down in the scale. Man appears to 

 be the goal of creation ; but even within the Vertebrate 

 series, every step of development, say of the Fish, is away 

 from the goal. The highest Fish is the one farthest from 

 Man. 



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. 125 Animals 

 like the Amoeba, with no cellular tissues nor true eggs, 

 form the subkingdom Protozoa. Animals like the Sponge, 

 with independent cells, one excurrent and many incurrent 

 openings, form the subkingdom Sjpongida. Animals like 

 the Coral, unlike all others, have an alimentary canal but 

 no body -cavity, have no separate nervous and vascular 

 regions, and the parts of the body radiate from a centre. 

 Such form a subkingdom called Codenterata. Animals 

 like the Star-fish, having also a radiating body, but a closed 



