A STUDY OF METAMERISM. 469 



pretation of the symmetry of organic form as the expression of 

 physical laws of protoplasm, rather than due to slow adapta- 

 tion of an amorphous or irregular substance to its surround- 

 ings^ is too large a question to attempt to discuss. Whatever 

 the explanation may be, there are certain well-established facts 

 in this connection that have, it seems to me, an important 

 bearing on metameric repetition. 



The relation existing between bilateral and radial symmetry 

 is one of the most suggestive fields of promorphology. We 

 get from this source more suggestion as to a possible interpre- 

 tation of the facts of metameric repetition than from any other 

 source. It would lead too far to attempt anything like a full 

 discussion, but I may cite two cases that will serve as simple 

 illustrations of a large field of inquiry. 



The radial symmetry of a sea-urchin is a very perfect type of 

 five-rayed structure. The test, made up of a mosaic work of 

 calcareous plates, is a marvellous piece of detailed fittiug. Yet 

 there are several cases on record of individuals that have a 

 sixth ray (antimere) introduced. Each of the six antimeres 

 may be a perfect copy of the others, as well as of the normal.^ 

 The same condition is not uncommon in the star-fish and other 

 Echinoderms, but owing to the lack of a mosaic calcareous 

 skeleton the result is not so impressive. 



Again, in other individuals one of the five antimeres may be 

 entirely or in part omitted, and yet the surface shows a perfect 

 symmetry. The point here to be emphasised is that a whole 

 section of the body (antimere) may be introduced or omitted, 

 involving the introduction or loss of all the organs belonging 

 to such a division. 



More remarkable still are the triangular tapeworms de- 

 scribed by Leuckart and others. A strongly marked bilateral 

 animal repeats occasionally one of its halves, so that we may 

 paradoxically speak of the worm as composed of three halves. 

 Instead of a somewhat flattened bilateral animal, there results 



' Bateson points out two cases which are to be distinguished. There may 

 be a division into two of one antimere, or there may be a redistribution of 

 the whole material into six parts. Tiie text refers to the latter. 



