No.1] 222 DEVELOPMENT OF BALANOGLOSSUS. 21 
stage it had taken for the first time sand into its digestive tract. 
The most conspicuous change in the present form is the greater 
elongation of the body, while the diameter of the body has 
either not increased at all or only slightly so. The proboscis 
has enlarged and assumed more of its adult proportions. Two 
large pigment spots are found at the apex. 
The collar has greatly altered. This is due to its elongation 
in the first place, and in the second to the closure of its median 
dorsal groove. This means that the central nervous system has 
sunken beneath the ectoderm of the collar, and that the collar 
has completely fused from side to side, obliterating all traces of 
its line of union. The circular groove near the posterior end of 
the collar is quite conspicuous, and this also is a further approach 
to the adult conditions. Beneath the posterior (overhanging) 
edge of the collar will be seen a pair of circular areas that 
indicate the position of the collar pores. These are seen 
through the walls of the collar, and their external opening lies 
beneath and behind the posterior edge of the collar and cannot 
in this position be seen at the surface. If the worm crawls 
directly away (down) from the observer these may often be 
seen as a pair of circular openings. 
The body behind the collar has greatly elongated. At its 
posterior end it swells up and becomes more spherical. This 
was scarcely noticed in the preceding stage, but in all later 
stages it is very apparent and is not an accidental condition 
due to temporary contraction or expansion of the body. What 
the function of this knob may be I do not know. 
The position of the pigment band is accurately shown and 
it will be seen that the growth of the body has taken place 
about equally in front of and behind the pigment band. It 
will also be noticed that the pigment band itself has become 
a little irregular. A slight longitudinal groove runs along the 
median dorsal line of the trunk and disappears beneath the collar 
anteriorly. This is not seen in the figure. | 
Behind the collar the gill region is conspicuous. Three 
pairs of gill-slits open at the surface ; the anterior is the largest, 
and the posterior is quite small as yet. Into each projects 
from the median and dorsal surface a bar that partly fills 
