circulation, the Mollusks, there are those that have no heart at all.  It would carry me too far into scientific details, were I to explain all the points at which this celebrated classification fails.  Suffice it to say that there is no better proof of the discrepancy between the system and the facts than the constant changes in the different editions of Oken’s own works and in the publications of his followers founded upon his views, showing that they were themselves conscious of the shifting and unstable character of their scientific ground.

VI.

What, then, is the relation of these larger groups to each other, if they do not stand in a connected 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 compared with each other.