A STUDY IN MORPHOLOGY. 127 



spending organs of allied animals which OWEN has proposed to distinguish by the 

 term " special homology." 



The structural relation between one appendage of Lucifer, say the first pereiopod, 

 and another, such as the swimmeret, is identical with the relation between the pereio- 

 pod of Lucifer and that of Squilla, or a Crab or Lobster. In both cases we have a 

 fundamental similarity of plan, which is independent of external conditions; and joined 

 to this essential similarity, we have a more superficial diversity of structure which is 

 plainly due to difference in the functions of the appendages, and their relations to the 

 external world. The resemblance between the two kinds of homology does not stop 

 here. Tracing the ontogeny of the appendages we find that there is much less differ- 

 ence between the larval pereiopods of Lucifer and those of the Lobster than there is 

 between the appendages of the adults, and we find exactly the same thing when we 

 compare the pereiopod and swimmeret of the same individual at earlier and earlier 

 stages of development. 



There is precisely the same resemblance between symmetry and special homology. 

 The right and left claws of the Common Crab (Callinectes) are not exactly alike, since 

 the cutting edge of one claw is sharp and set with pointed teeth, while the edge of 

 the other is thick, with thick blunt crushing tubercles. The two appendages are alike 

 in plan or homologous, but each is fitted for a specialised function by a slight structural 

 peculiarity. In this case, as in the others, the differences are less marked, and the 

 common plan more closely followed, in the larva than in the adult. 



Serial homology and bilateral symmetry are thus seen to be like special homology 

 in all purely structural features. In each case the homology is a resemblance which 

 is independent of external conditions, but which may be obscured by secondary modifi- 

 cations whenever external conditions render it necessary. 



In each case, too, the secondary modifications become less marked, and the underlying- 

 plan more evident as we pass back from the adult to earlier and earlier stages of deve- 

 lopment. We must therefore include ah 1 three kinds of homology in a single class or 

 category, and the employment of one term to denote the phenomena of special homo- 

 logy, of another for serial hornologies, and a third for bilateral homologies, and others for 

 other sorts of general homology must not be allowed to obscure the fact that they are 

 all different forms of the same thing, essential similarity joined to superficial diversity. 

 The terminology which has been employed by BRONN, H^ECKEL, LANKESTER, and 

 others for the different kinds of homology is valuable, and the only reason why I 

 have not made use of it is that the more familiar terms, "serial homology" and "bi-lateral 

 symmetry" answer every purpose equally well in treating of the Arthropods. H.-ECKEL'S 

 subdivisions are natural, but they are simply subdivisions of a great class of similar 

 phenomena, which must still be included under the general term " homology." 



Special homology may be defined in two ways, morphologically and phylogenetically. 

 From the morphological point of view an homology is a similarity in essential plan of 

 structure, which may be obscured by differences due to diversity of function. From 



