THE CATEGORIES OF ANALOGY. 277 



cal forms cannot fail to add precision to our zoological 

 investigations. When, for instance, the form of the Worms 

 is compared with that of the Holothurians, it should be 

 borne in mind that in the Worms, according to the plan of 

 their structure and their homology to the other Articulates, 

 their longer diameter is the longitudinal diameter; while 

 the longer diameter of the Holothurians, when identified 

 by then 1 homologies with the other Radiates, is their ver- 

 tical diameter. This shews at once, that however similar 

 to 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 the 

 limits 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 similar 

 forms within the same class are not necessarily homolo- 

 gous, for instance, the long snout of Syngnathus, and 

 that of Fistularia, or the flat heads of Lophius and of 

 Scaphirhynchus, are only remotely analogous, their struc- 

 ture being entirely different. The forms of animals have 

 been so imperfectly studied, and the structural elements 

 which determine them so little considered, that the time 

 has hardly come yet to determine with any degree of 

 accuracy the analogies and homologies of the form of ani- 

 mals. Considered with reference to their position, the 

 six pairs of articulated appendages which are placed upon 

 the 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 the 

 latter; and yet, as appendages to the normal rings of an 

 Articulate, all these parts are homologous. Here, there- 

 fore, it becomes necessary to remember that while the 



