METAMORPHOSIS OF ACTINOTROCHA, 209 
garding the creature as a true Gephyrean larva. If the 
circlet of swimming arms were reduced to a simple postoral 
belt and the preoral lobe reduced in size, the larva would 
agree fairly well with the larva of Phascolosoma or Bonellia. 
If, in support of this view, it is necessary to consider 
the longer cilia along the margin of the preoral lobe as 
representing a preoral belt comparable to that of the 
Echiurus larva, I see no valid objection to so considering 
them. 
Accepting, then, the view that Phoronis is a true Gephy- 
rean, we may note that among these animals two series of 
forms may be distinguished, which differ from each other in 
the course of the alimentary canal and the position of the 
anus. This division of the group is not necessarily a 
*‘natural” one; for our present purpose this point is quite 
immaterial. In one series, represented by such forms as 
Bonellia, Thalassema, Hehiurus, the mouth and anus are at 
opposite extremities of the body, and the alimentary canal, 
aside from secondary flexures, pursues a straight course 
from one to the other. In the other series, including 
Phascolosoma, Sipunculus, Phoronis, and others, the anus 
is near the mouth on the apparently dorsal side of the body ; 
and the alimentary canal is bent upon itself to form a long 
U-shaped loop, which is usually complicated by secondary 
flexures, as in the first series. The <Actinotrochan larva of 
Phoronis, aside from its specially acquired swimming appa- 
ratus, corresponds pretty closely with such a form as Thalas- 
sema, representing the first series. And the larva may itself 
fairly be taken as a representative of this series. The 
presence of swimming arms is no obstacle to this view, 
for these are, clearly, provisions to increase the surface 
for the attachment of locomotor cilia, and can have no 
ancestral significance.! 
Through its metamorphosis the larva becomes an extreme 
representative of the second series, the anus undergoing, 
apparently, a transfer from one extremity of the body toa 
point very near the opposite extremity, beside the mouth, 
and the intestine acquiring a corresponding flexure. 
From these and other more general considerations, it is 
evident that the displacement of the anus and corresponding 
flexure among the Gephyrea in general are derivative cha- 
racters which, in all probability, were originally acquired to 
1 Professor Lankester’s comparison of these arms to the “ branchio- 
troch ” of Echinoderm larve and other forms can be valid only if taken in 
the most general sense, as was no doubt intended, and not as indicative of 
any special relationship to those forms, 
