10 
Siphonophora rosarum 
ganglia to the muscles of the mouth-organs, as is the case in the 
apterous form. This, however, does not disprove that such a nerve is 
present, and moreover the nerve in the apterous form is very fine, and 
not easily traced in transverse sections. It is probable that its minuteness 
has prevented its discovery. 
The thoracic ganglia. At first sight it would seem as if there was 
very little difference in this part of the nervous system. The thoracic 
ganglia are connected by a similar elongated portion with the sub- 
oesophageal ganglia, and a pair of ganglia are present for each of the 
three segments of the thorax. The remarkable feature about the ganglia 
is the concentration which has undoubtedly taken place from the typical 
nervous system, so that the whole of the thoracic ganglia and the 
median abdominal ganglion lie almost entirely in the prothoracic 
segment, extending but a short distance into the mesothorax. The 
consequence of this crowding is that the ganglia lie partially one above 
another; so that the posterior portion of the first thoracic ganglia lies 
underneath the anterior portion of the second thoracic ganglia. The 
second and third thoracic ganglia lie in like relation. 
If the view be taken, and it is the most probable one, that the 
apterous stage is a degeneration from the winged stage, it will appear 
that the elimination of the habit of flight has merely caused a slight 
step back towards the type-form of nervous system, in that the ganglia 
in the apterous stage are not so crowded together, and that a gradual 
relative elongation of the system has taken place. The crowding of the 
ganglia in the winged stage is therefore probably due to some evolu¬ 
tionary influence, and may not have any connection with the presence of 
organs of flight. It must, however, be borne in mind that from the 
point of view of the nervous system, it is just as feasible to suppose that 
the winged form is an advance on the apterous form, and that the intro¬ 
duction of the habit of flight has made greater demands on the thoracic 
ganglia, and that the increase in size necessitated by this increased 
demand has resulted in the crowding of the ganglia together. 
The nerves from the thoracic ganglia are distributed in a different 
manner from that described in the apterous form. This is due to 
the extraordinary development of the mesothorax to accommodate the 
wing muscles. The effect of this enlargement is that the first and 
second pairs of legs are much further apart than are the second and 
third pairs. Consequently, while the nerve from each of the first 
thoracic ganglia leaves its ganglion almost at right angles to the axis of 
the body, the nerves from the second and third thoracic ganglia are 
