No.1.] THE DEVELOPMENT OF BALANOGLOSSUS. 19 
circular band is rendered conspicuous by a narrow and some- 
what irregular circle of pigment, and projecting from the band 
are to be found short flimmering cilia. The sand immediately 
around the larva is arranged into a tenacious tube ; the parti- 
cles held together by a mucus secreted by the animal. The 
young worm moves from place to place by a characteristic 
peristaltic movement, as in the adult. 
If we examine somewhat more in detail the different regions 
of the body we find that the proboscis has become more 
elongated. At its base is found the opening of the anterior 
body-cavity. 
The collar is much broader than in the preceding figure and 
more sharply separated from the body behind it. In the 
median dorsal line a longitudinal groove separates the two 
sides of the collar, and sections show that here the collar is 
rolling over from the sides to cover in the median piece of 
ectoderm that becomes the central nervous system. 
The greatest change has taken place in the body behind the 
collar, vzz., in its increase in length. We have, fortunately, a 
well-defined and important landmark by which to gauge the 
growth of this region. The circular ciliated band is con- 
spicuous on account of its dark pigment, ‘and we are thus 
able to measure the relative growth of the areas before and 
behind the band. We find that the two regions have increased 
to nearly the same extent ; the posterior is a little longer, but 
this difference was also seen in the preceding figure (Fig. 6). 
In the anterior region of the body and behind the collar we 
find on the dorsal surface two pairs of large round openings. 
The first pair is the larger, and lies behind and to some extent 
beneath the rim of the collar ; and the second pair, which is 
smaller, lies immediately behind the first pair. These are 
the external openings of the first and second gill-slits. In 
other words the first and second gill-pouches of the preceding 
stage have opened to the exterior by four openings. 
The openings of the collar pores were not seen in the living 
larva, but surface views of preserved larvae and serial sections 
show that collar pores lie near to the first pair of gill-slits. 
The collar pores, we have seen, arise in point of time before 
