14 MORGAN. Vor. TX. 
region, as we shall see, there is developed later not only the 
collar and its peculiar organs, but the anterior end of the body 
proper as well, involving the whole of the gill region. 
The posterior ciliated band has grown broader, and is marked 
by dark, black pigment specks. The cilia are larger and 
thicker, and by their movements drive the larva through the 
water. Larvae at this stage were repeatedly caught in the 
surface nets, and it is from a larva just caught that the 
present figure was drawn. A small secondary circular band 
is found around the bulging posterior plate, and may be readily 
seen in sections and preserved larvae (it is not shown in the 
figure, as it was overlooked in the living animal). 
In connection with the decrease in size of the body as a 
whole, we find the ectodermal walls considerably thickened 
and pigment spots scattered over the surface. The circum- 
oral area has decreased in surface exposure more proportionally 
than the extra-oral area, and its walls are correspondingly thick- 
ened. Owing to these changes the animal is less transparent 
than in the preceding stages. The large, brown, crescentic 
eyes are conspicuous at the apex of the larva. 
If we examine the internal organs we find the most important 
change to be the shifting of the alimentary canal, noticed 
above. On each side of the oesophagus we find three protru- 
sions, as shown in the figure. The anterior is the largest, the 
third is just forming. These six protrusions are the beginning 
of the gill-pouches. It may be stated that they now lie at a 
level far anterior to the region where they are subsequently 
to open to the exterior. This fact indicates that the digestive 
tract is to undergo farther changes before it gets into its final 
position. 
The anterior body-cavity has enlarged, and may be dimly 
seen through the walls of the body. It fills up a large part 
of the interior of the anterior end of the larva, and opens by 
the dorsal pore in front of the constriction and a little to the 
left of the median line. Near its external opening the irregular 
outline of the proboscis-vesicle is seen projecting into the 
interior of the body-cavity at this point. A very short string 
of cells connects the body-cavity with the under surface of the 
