STRUCTURE AND CLASSIFICATION OF THE AETHKOPODA. 535 



tliat to which the wandering merome belongs in origin. The 

 movement is more usual from behind forwards than in the 

 reverse direction ; but this probably has no profound signifi- 

 cance, and depends simply on the fact that, as a rule, the head 

 must be the chief region of development on account of its 

 containing' the sense organs and the mouth. 



In the Vertebrata the independence of the meromes is more 

 fnlly developed than in other metamerised animals. Not only 

 do we get auto-heterosis of the meromes on a most extensive 

 scale, but the dislocation of single meromes and of whole 

 series (tagmata) of meromes is a common phenomenon. Thus 

 in fishes the pelvic fins may travel forwards to a thoracic and 

 even jugular position in front of the pectoral fins; the 

 branchiomeromes lose all relation to the position of the 

 meromes of muscular, skeletal, ccelomic, and nervous nature, 

 and the heart and its vessels may move backwards from their 

 original metameres in higher Vertebrates carrying nerve-loops 

 "with them. 



The Seventh Law of metamerism is one which has been 

 pointed out to the writer by Mr. E. S. Goodrich, of Merton 

 College, Oxford. It may be called the law of " translation 

 of heterosis." Whilst actual physical transference of the 

 substance of meromes undeniably takes place in such a case 

 as the passage of the pelvic fins of some fishes to the front 

 of the pectorals, and in the case of the backward movement 

 of the opisthosoinatic appendages of spiders, yet the more 

 frequent mode in which an alteration in the position of a 

 specialised organ in the series or scale of metameres takes 

 place is not by migration of the actual material organ from 

 somite to somite, but by translation of the quality or 

 morphogenetic peculiarity from somite to somite accompanied 

 by correlative change in all the somites of the series. The 

 phenomenon may be compared to the transposition of a piece 

 of music to a higher or lower key. It is thus that the lateral 

 fins of fishes move up and down the scale of vertebral somites; ^ 

 and thus that whole regions (tagmata), such as those indicated 

 * Except in such cases as liave just been cited. — E. R. L., 1901. 



