A STUDY OF METAMERISM. 425 



below this line only extends a third across. Whether this 

 would ultimately form a complete ring, or whether we have 

 here the beginning of a spiral starting with a half-compound 

 metamere below, is uncertain. 



Several vertical longitudinal sections were made through 

 these " tails/' but no additional information was gained from 

 the sections. The irregularities in the positions of the rudi- 

 mentary body cavities was very apparent. 



VI. Modifications of Internal Structures. 



Before any conclusion can be reached as to the value of the 

 irregularities seen on the surface, we must examine the condi- 

 tion of the internal organs, particularly the arrangement of 

 the septa. 



We can formulate this general statement, that the ar- 

 rangement of the septa generally conforms to the 

 curves of the lines seen on the surface. This means 

 that the phenomena are deep-seated, and that all the struc- 

 tures of the body are involved in the new arrangement, and 

 not merely the external surface lines. 



Exceptional cases are not uncommon, in which we find two 

 principal departures from the rule. First, the arrangement of 

 the septa does not always agree with the surface lines. Some- 

 times the arrangement of the septa is more perfect, and some- 

 times simply different. Secondly, the arrangement of the 

 somatic attachment of the septa is sometimes different from 

 the splanchnic attachment. Usually in this case the somatic 

 portions of the septa follow the lines found on the surface, 

 while the splanchnic attachment differs in its arrangement, 

 and is usually more irregular in its form. 



If a worm having a compound metamere be opened we find 

 the septa arranged as shown in fig. 61, a, b. One septum (s) 

 reaches only to the mid-dorsal line. If we remove the diges- 

 tive tract we find the same septum ending freely below, just 

 before reaching the middle line (s'). This half-septum is found 

 to correspond to the "split" in the compound metamere, and 

 lies between the two half-metameres of that side. 



