ON PHOBONIS OVALIS, STBE THILL WRIGHT. 
137 
be an unusual condition. The position of the fission-zone 
which cut off the small individual shown is clearly indicated 
by the annular tube-septum. But in this case the regenerating 
distal end is growing into the part of the original tube situated 
distally to the septum, instead of growing out laterally to 
form a completely new opening. Perhaps the septum was not 
a complete one ; but if not it must be assumed that the central 
part of the septum has been absorbed by the regenerating 
fragment. It is not more difficult to make this assumption than 
to assume that in other cases a lateral part of the tube can be 
absorbed, in order to allow the proximal fission-segment to 
form a new orifice to its tube. 
The great capacity for transverse fission possessed by 
P. oval is is indicated by the very small size of the re- 
generating fragments. The smallest specimen shown (PI. 7, 
fig. 7) is only *3 mm. long, but it shows clear signs of re- 
generation in the differentiation of a new muscular region of 
the body-wall, indicated by a greater thickness of this part 
distally, and by the formation of a distinct line of separation 
between it and the future non-muscular portion. The appear- 
ances here shown give reason to suppose that a fragment no 
more than *3 mm. long can regenerate a complete individual. 
The complicated arrangement of the tube-septa in this case 
implies that the cavity of the original tube has been reduced 
in size several times, probably in correlation with the small 
size of the living fragment left in this section of the tube. 
(e) Position of the zones of fission. 
The direct evidence obtained on this subject points to the 
non-muscular part of the body as the region where fission 
may occur. This is illustrated by PI. 9, figs. 29-31. It may 
be noted that this is not in agreement with the statement of 
de Selys-Longcliamps (1907, p. 163), according to whom it is 
the muscular region that is specially capable of regeneration. 
The observation by this author that, having cut the muscular 
region of a Phoronis psammophila into six fragments, 
