16 BULLETIN OF THE 
gradually to move away from the neck of the polypide at which they 
arose, in order finally to lie so that the muscle fibres are nearly parallel ? 
If the points of origin remain fixed with reference to the surrounding 
cells, they can hardly come to lie absolutely closer together, but only 
relatively so by growth of the body wall between these points and the 
neck. If, however, we find that in older individuals the points of origin 
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 behind 
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- N°1- No.2 No.8. No.4, 
ClasyAtial Aldo urn «25 cea iia th. s ELOU 154 187 260 
Distance on periphery between origins of mus- 
cles, abatrlal Bildes sei) ay ey ie), oe OOF 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” ofsthe 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 29.1) 
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. 
