In Mollusks there is a longitudinal axis and a bilateral symmetry; but the longitudinal axis in these soft concentrated bodies is not very prominent; and though the two ends of this axis are distinct from each other, the difference is not so marked that we can say at once, for all of them, which is the anterior and which the posterior extremity.  In this type, right and left have the preponderance over the other diameters of the body.  The sides are the prominent parts,—­they are charged with the important organs, loaded with those peculiarities of the structure that give it character.  The Oyster is a good instance of this, with its double valve, so swollen on one side, so flat on the other.  There is an unconscious recognition of this in the arrangement of all collections of Mollusks; for, though the collectors do not put up their specimens with any intention of illustrating this peculiarity, they instinctively give them the position best calculated to display their distinctive characteristics, and to accomplish this they necessarily place them in such a manner as to show the sides.  In Articulates there is also a longitudinal axis of the body and a bilateral symmetry in the arrangement of parts; the head and tail are marked, and the right and left sides are distinct.  But the prominent tendency in this type is the development of the dorsal and ventral region; here above and below prevail over right and left.  It is the back and the lower side that have the preponderance over any other part of the structure in Articulates.  The body is divided from end to end by a succession of transverse constrictions, forming movable rings; but the character of the animal, its striking features, are always above or below, and especially developed on the back.  Any collection of Insects or Crustacea is an evidence of this; being always instinctively arranged in such a manner as to show the predominant features, they uniformly exhibit the back of the animal.  The profile view of an Articulate has no significance; whereas in a Mollusk, on the contrary, the profile view is the most illustrative of the structural character.  In the highest division, the Vertebrates, so characteristically called by Baer the Doubly Symmetrical type, a solid column runs through the body with an arch above and an arch below, thus forming a double internal cavity.  In this type, the head is the prominent feature; it is, as it were, the loaded end of the longitudinal axis, so charged with vitality as to form an intelligent brain, and rising in man to such predominance as to command and control the whole organism.  The structure is arranged above and below this axis, the upper cavity containing all the sensitive organs, and the lower cavity containing all those by which life is maintained.