10 MORGAN. [Vo.. IX. 
In the interior of the larva is seen the digestive tract, with 
its three divisions of oesophagus, stomach, and intestine. A 
part of the posterior plate has been pulled in somewhat, form- 
ing what looks like a fourth division of the digestive tract ; 
this is only accidental. 
The anterior body-cavity runs from the anterior surface of 
the digestive tract to a point near the mid-dorsal line, where it 
opens to the exterior by a small pore. A delicate chord of 
cells connects the anterior end of this body-cavity with the 
under surface of the apical plate. This chord of cells is to 
be looked upon as a diverticulum from the body-cavity rather 
than a muscle-band lying outside the body-cavity. 
Serial sections show that, at this stage of the development, 
two important organs arise. Just above the opening of the 
water-pore a hollow sphere of cells is found, which is the begin- 
ning of the proboscis-vesicle. Its method of. origin will be 
described later. The other organs that appear are the pos- 
terior pair of body-cavities. This last pair—third pair of 
body-cavities — appears just beneath the circular band. The 
details of its origin are left for later treatment. 
We may pass next to the Tornaria, when it has reached 
the full perfection of its larval life. Fig. 3, Pl. I, is an 
attempt to reproduce the larva at this stage; but it would 
take a more elaborate drawing to do justice to the beauty of 
the larva itself. 
The most obvious differences between this and the pre- 
ceding figure are the great increase in size (4% mm. long), 
together with the growth in length of the tentacular processes. 
These stand out freely from the surface. The ciliated band 
runs up one side of each tentacle, crosses its top, and down the 
other side; thence over to the next tentacle. Each group 
of tentacles has approximately the same number of tentacles 
in it. 
Another important change is the relatively smaller exposure 
of the ‘circum-oral area compared with the extra-oral. It is 
due to the fact that the circum-oral area has sunken inwards 
from the general surface, so that it presents a concave face to 
