210 EDMUND B, WILSON, 
adapt the animals more perfectly to a tubicolous life. We 
have seen how this shifting of the anus is effected in the 
individual development or ontogeny of the animal; and it is 
plain that in considering how this mothod of development ~ 
has been acquired, the first question to answer is as to how 
this shifting was effected in past time during the phylogeny, 
as the ancestors of the animal passed from a free-swimming, 
or at least wandering, mode of life to a strictly sedentary 
tubicolous one. 
The answers to this question are possible. The first is 
that the anus gradually moved dorsalwards, and then for- 
wards to its present position behind the mouth. The 
second, which is the view here maintained, is that the worm 
upon assuming the tubicolous mode of life developed a habit 
of bending the body into a U-shape, bringing the posterior 
part of the body alongside the anterior part in order to dis- 
charge the excreta at the mouth of the tube, or for some 
similar reason. External signs of this flexure becoming in 
time obliterated by coalescence of the two limbs of the U, 
the body once more assumed astraight linear form, but with 
the anus and mouth now near the same extremity. 
Obviously, there are many difficulties in the way of the 
supposition that the anus gradually moved forwards along 
the dorsal side. This process seems impossible if we as- 
sume the worm to have meanwhile retained its tubicolous 
habit ; for only a very great displacement could be of any 
advantage, and it is impossible to suppose that such 
a displacement could occur as a natural variation of struc- 
ture. To assume an intermediate mode of life during which 
the creature should live, for instance, in mud or sand, would 
not obviate this difficulty, but would do away with the need 
for any displacement at all. The existence of forms like 
Phascolosoma, which live in sand or mud, and yet have a 
greatly displaced anus, does not meet the objection; for 
there is some reason to believe that such forms may be 
considered descendants of strictly tubicolous forms which 
for some reason—perhaps increase of size—have abandoned 
their former mode of life. 
On the other hand, the flexure hypothesis has consider- 
able evidence in its favour. Among the tubicolous Anne- 
lides there are forms—e.g. Sabellaria—which habitually 
flex the body, as I suppose the ancestors of Phoronis to 
have done. Such a form is shown in fig. 20, which repre- 
sents a large Sabellaria from the North Carolina coast. It 
is very noteworthy that the direction of the flexure is con- 
stant and toward the dorsal aspect of the body; so that if in 
