THE CATEGORIES OF ANALOGY.

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cal forms cannot fail to add precision to onr zoologicalinvestigations. When, for instance, the form of the Worms is compared with that of the Holothurians , it should beborne in mind that in the Worms , according to the plan oftheir structure and their homology to the other Articulates,their longer diameter is the longitudinal diameter; whilethe longer diameter of the Holothurians , when identifiedby their homologies with the other Radiates, is their ver-tical diameter. This shews at once, that however similarto one another, the form of the Holothurians is only ana-logous to that of the Worms .

The limits within which similar forms may be homolo-gous appear to be very wide, and to extend beyond thelimits of their respective classes. The form of the Salaman­ ders and the Lizards, for instance, is certainly homological,though they are members of different classes; yet similarforms within the same class are not necessarily homolo-gous,—for instance, the long snout of Syngnathus, andthat of Fistularia, or the flat heads of Lophius and ofScaphirhynchus, are only remotely analogous, their struc-ture being entirely different. The forms of animals havebeen so imperfectly studied, and the structural elementswhich determine them so little considered, that the timehas hardly come yet to determine with any degree ofaccuracy the analogies and homologies of the form of ani-mals. Considered with reference to their position, thesix pairs of articulated appendages which are placed uponthe sides of the mouth of the horse-shoe crab (Limulus)are truly homologous to the jaws of the higher Crustacea;but by their form they resemble the thoracic legs of thelatter; and yet, as appendages to the normal rings of anArticulate, all these parts are homologous. Here, there-fore, it becomes necessary to remember that while the