METAMORPHOSIS OF ACTINOTROCHA, 211 
time external signs of the flexure should disappear, the 
anus would have exactly the same position it occupies in 
Phoronis, and the intestine would have the same disposi- 
tion. Admitting that such a flexure took place, it seems 
very probable, @ prior, that the two parts of the body thus 
brought into proximity would tend in time to coalesce, and 
thus to destroy the appearance of flexure. In certain ana- 
logous cases such a flexure and fusion plainly seems to 
have occurred. A clear and convincing illustration is af- 
forded by certain Holothurians; for here a record of the 
change has been preserved in the rows of ambulacral 
suckers. These retain their flexed course, and thus give 
us external evidences of the history of the animal, which 
are wanting in Phoronis. Such forms as Pentacta or 
Synapta are perfectly straight, exhibit a striking though 
not absolutely perfect radial symmetry, and the radii 
of the body are of equal length. In other forms, like 
Cucumaria, which, living in sand, habitually bend the body 
sharply in the middle to bring both extremities to the sur- 
face, the body has lost something of its radial symmetry. 
The side of the body towards which the flexure is made 
is perceptibly shorter than the other, and the body cannot 
be fully straightened. In other words, the flexure is parti- 
ally obliterated by coalescence of the body walls, it has 
become organic. Here, again, the direction of the flexure 
is constant, being towards the bivium. A further step is 
seen in forms like Psolus or Lophothuria, where the radial 
symmetry has become profoundly modified, and the mouth 
and cloaca are perhaps twice as far apart on the side of the 
trivium as on the opposite side. Whether the next stage, 
which would be in shape something like an Ascidian, exists 
among the Holothurians I do not know. But the extreme 
of the series is found in Rhopalodina, where the mouth and 
cloacal opening are in immediate proximity, and the body 
is considerably elongated in a direction transverse to the 
original long axis. That this structure was really brought 
about by the complete flexure of a primitiveiy straight and 
radially symmetrical form is shown by the persistence of 
this flexure, almost unaltered in the ambulacral vessels, at 
first pointed out by Hermann Ludwig. 
Another illustration is afforded by the Chetopod larva 
Mitraria. In this well-known form the digestive tube is 
completely doubled upon itself, so that the mouth and anus 
are in immediate proximity. We must believe that here, 
too, the entire body has undergone an actual or virtual 
flexure corresponding to that of the intestine ; for in a later 
