ON THE DIPLOCHORDA. 481 
On the Diplochorda. 
By 
A. T. Masterman, M.A., D.Sc. 
With Plate 29. 
Part V.—Certain Points in the Structure of Tornaria. 
Dorine the summer of 1905 I paid a visit to the Biological 
Station at Heligoland with the object of obtaining materials 
for a renewed study of the development of Phoronis. I was 
at the same time enabled to examine living larve of Kchino- 
derms and a few Tornaria. 
Preliminary results upon certain points in the structure of 
these larvee were indicated in 1898, but until 1905 I was 
unable to obtain sufficient material for definite conclusions. 
One of the most detailed descriptions of Tornaria at 
various stages is that of Morgan,! who has also followed its 
development into the adult, in the case of at least two species ; 
I have had occasion to make constant reference to his results. 
It may be recalled that the general surface of Tornaria is 
even more sharply divided into two areas than that of Echino- 
derm pelagic larvee, by the sinuous course of a ciliated band 
“usually known as the “circum-oral band” (Morgan). The 
area lying within this band is known as the circum-oral area 
in contradistinction to the remainder or the extra-oral area. 
The circum-oral area is very thin-walled, and is, at least in 
older stages of Tornaria, more or less sunk inwards below 
the level of the thicker-walled extra-oral area. It is bounded 
1 Morgan, T. H., ‘‘ Journal of Morphology,’ vols. v and ix. 
