THE CLASSIFICATION OF ANIMALS. 
233 
and Invertebrates—is partially bridged on the one side by 
Amphioxus, and on the other by Balanoglossus 'a worm¬ 
like animal) and 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. 1 * 6 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 snbkingdom Porifera. 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 Ccelenterata. Animals 
like the Star-fish, having also a radiating body, but a closed 
