16 BULLETIN OF THE 



gradually to move away from the neck of the polypide at which thoy 

 arose, in order finally to lie so that the muscle fibres are neai'ly parallel ] 

 If the points of origin remain fixed with reference to the surrounding 

 cells, they can hardly come to lie ahsolately closer together, but only 

 relatively so by growth of the hody wall between these points and the 

 neck. If, however, we find that in older individuals the points of oi'igin 

 are not only relatively but absolutely closer together, we are driven to 

 the conclusion that these points move relatively to the surrounding cells. 

 To decide whether the points of origin come to lie closer together hehind 

 the neck absolutely or only relatively, I measured cross sections of four 

 individuals through the region of the neck in which the muscle fibres 

 showed evident differences of length, and therefore of age. I may 

 preface a table of these measurements with the statement that the mus- 

 cles first appear plainly differentiated at a stage when the polypide is 

 well formed (Fig. 7, mu. pyr.), and that the growth of the body wall in 

 circumference is not very considerable after this time. The numbers 

 indicate measurements in micra : — 



Distance on periphery between origins of mus- °' °' ' 



cles, atrial side 150 154 187 260 



Distance on periphery between origins of mus- 

 cles, abatrial side 297 286 264 220 



Total length of periphery 447 440 451 480 



The distance on the "atrial side" signifies the distance measured over 

 a, b, c, Figure 63 (Plate VI.). The length of the remainder of the sec- 

 tion is the distance on the " abatrial side." 



From these measurements it appears that the " origins " of the py- 

 ramidal muscles approach each other absolutely, — a condition which 

 Braem's hypothesis cannot explain, and which can be reasonably inter- 

 preted, it seems to me, only by assuming, however unique and difficult 

 of conception such a condition may be, that the points of origin move 

 relatively to the surrounding cells of the body wall. (Compare also 

 the movement of parietal muscles referred to on page 29.^) 



It is not necessary to assume that the increase in extent of the body 

 wall after the polypide is first formed is due to the addition of cells from 



1 Professor Mark has called my attention to a discussion of the movement of 

 the fixation-point of a muscle in Mollusks by Tullberg ('82, pp. 26, 27, 44). This 

 author says that he has undertaken no special investigation of the method of 

 migration, but concludes that this motion must result from the absorption of the 

 inner muscle fibres as new ones are formed on the outside. I do not find any 

 evidence of such a process in Paludicella. 



