STRUCTURE AND DEVELOPMENT OP LOXOSOMA. 297 
anteriorly has given off a solid outgrowth which has met one 
end of the brain. This union of an outgrowth from each 
vestibular invagination with an end of the crescent- shaped 
brain is a constant occurrence ; and it may be concluded that 
some of the deeper cells of the vestibular invaginations sepa- 
rate from the external epiblast to form a pair of suboesophageal 
ganglia, united with the brain (and with one another) by 
means of a circumcesophageal commissure. It must, however, 
be distinctly understood that the suboesophageal ganglion 
arises from the deeper cells of an epiblastic thickening, and 
not by a direct conversion of the vestibular invaginations into 
nervous tissue. The formation of the ganglion in the bud 
thus takes place in exactly the same manner as that of its 
homologue in the embryo. In stages more advanced than 
fig. 54, it is still possible to discover a commissure running 
from the brain on each side to a mass of tissue in the post-oral 
region, although at no period is it at all easy to observe the 
limits of the suboesophageal ganglion, which is closely packed 
with mesoblast cells, very difficult to distinguish in section 
from ganglion-cells. 
The dorsal organ of L ox o soma, as is well known, is pro- 
vided with a pair of ciliated sacs, one on each side, in the 
neighbourhood of the eyes, and opening to the exterior on its 
surface. These sacs are provided with active cilia, and extend 
inwards so as to lie in close contact with the external surface 
of the brain. As far as I am aware, the presence of these two 
sacs is the only foundation for the statement in Yol. i of 
Balfour’s f Comparative Embryology/ that the dorsal organ of 
Loxosoma is double, whereas that of Pedicellina is single. 
The brain of Loxosoma is in reality developed from an 
unpaired rudiment, and except for the fact that it extends 
further laterally than that of Pedicellina, there is no reason 
whatever for assuming its double nature. In Pedicellina 
the lumen of the brain atrophies, as already described by 
Hatschek ; but before this process has taken place, there occurs, 
in the same position as in Loxosoma, a deposition of fibrous 
tissue, as I have been able to observe from sections. The 
