cted series from the lowest to the highest?
How far are each of the branches and each of the classes superior or
inferior one to another? All agree, that, while Vertebrates stand at the
head of the Animal Kingdom, Radiates are lowest. There can be no doubt
upon this point; for, while the Vertebrate plan, founded upon a double
symmetry, includes the highest possibilities of animal organization,
there is a certain monotony of structure in the Radiate plan, in which
the body is divided into a number of identical parts, bearing definite
relations to a central vertical axis. But while all admit that
Vertebrates are highest and Radiates lowest, how do the Articulates and
Mollusks stand to these and to each other? To me it seems, that, while
both are decidedly superior to the Radiates and inferior to the
Vertebrates, we cannot predicate absolute superiority or inferiority of
organization of either of these groups as compared with each other; they
stand on one structural level, though with different tendencies,--the
body in Mollusks having always a soft, massive, concentrated character,
with great power of contraction and dilatation, while the body in
Articulates has nothing of this compactness and concentration, but on
the contrary is usually marked by a conspicuous external display of
limbs and other appendages, and by a remarkable elongation of the
body,--that feature characterized by Baer when he called them the
Longitudinal type. There is in the Articulates an extraordinary tendency
toward outward expression, singularly in contrast to the soft,
contractile bodies of the Mollusks. We need only remember the numerous
Insects with small bodies and enormously long wings, or the Spiders with
little bodies and long legs, or the number and length of the claws in
the Lobsters and Crabs, as illustrations of this statement for the
Articulates, while the soft compact body of the Oyster or of the Snail
is equally characteristic of the Mollusks; and though it may seem that
this assertion cannot apply to the highest class of Mollusks, the
Cephalopoda, including the Cuttle-Fishes with their long arms or
feelers, yet even these conspicuous appendages have considerable power
of contraction and dilatation, and in the Nautili may even be drawn
completely within the shell. If this view be correct, these two types
occupy an intermediate position between the highest and the lowest
divisions of the Animal Kingdom, but are on equal ground when compa