470 T. H. MORGAN. 



a radial (triradiate) form, having three sets of longitudinal 

 organs where normally there are only two. Two other facts 

 in connection with these variations ought to be emphasised. 

 The scolices of the tapeworms arise on the large bladderworm 

 by invagination of the surface wall. The relation of the inva- 

 gination to the surface of the sphere is a radial rather than a 

 bilateral relation, and it is interesting to find this occasionally 

 expressed in the triangular scolices. In the second place, 

 Leuckart records finding on the same bladder (Cysticercus) 

 both radial and bilateral scolices. This seems to show that the 

 radial type need not have come in from egg variation, but is 

 the result of conditions acting at the time of the formation of 

 the scolices. 



Now both of these cases, the sea-urchin and the triangular 

 tapeworms, show that a complete section of the body may be 

 repeated and intercalated symmetrically amongst the other 

 parts. The new portion has appeared at once and fully 

 equipped, duplicating the structures of the body that lie in a 

 similar axial position. 



These and many other similar cases show us, I think, very 

 positively that the variations appearing in a radial animal 

 must have come simultaneously and all together into the anti- 

 meres. 



Moreover I think no one will doubt that the relation 

 existing between the repeated organs in a radiate animal is at 

 bottom the same relation existing between the right and left 

 sides of the body of a bilateral animal. 



Mivart (31) and Brooks (9) have emphasised the further fact 

 that the relation between the right and left sides of the body 

 is the same relation that exists between the serially repeated 

 parts of a metameric animal. 



If this line of argument be admitted, it puts the problem of 

 metamerism into a large category of well-established facts. 

 That the final explanations of these facts is closely bound up 

 with the solution of some of the most fundamental problems 

 of biology is self-evident. 



To hope, therefore, to solve the problem of metamerism in 



