THE EVIDENCE OF THE OLFACTORY APPARATUS 225 
sections on each side of the median one. The muscles of the pharynx 
and the muscles of the camerostome are attached to the pre-oral 
entosclerite (pr. ent.). The post-oral entosclerite is shown in section 
as post. ent. The dorsal blood-vessel, or heart, is indicated at H. 
In Fig. 97 I give a transverse section through another specimen 
of the same litter, to show the nature of this olfactory tube when cut 
across. Both sections show most clearly that we are dealing here with 
an elaborate sense-organ, the surface of which is partly covered with 
very fine long hairs, partly, as is seen in the figure, is composed of 
long, separate, closely-set sense-rods (dat.), well protected by the 
long hairs which project on every side in front of them, which recall 
to mind Bellonci’s figure of the ‘ batonnets olfactives’ on the antenne 
of Spheroma. Finally, we have the observation of Blanchard quoted 
by Huxley, to the effect that this camerostome is innervated by 
nerves from the supra-cesophageal ganglia which are clearly bilateral, 
seeing that they arise from the ganglion on each side and then unite 
to pass into the camerostome ; in other words, paired olfactory nerves 
from the supra-cesophageal ganglia. 
These facts demonstrate with wonderful clearness that in one 
group of the Arthropoda the olfactory antennz have been so modi- 
fied as to form an olfactory tube or passage, which leads directly 
into the mouth and so to the esophagus of the animal, and, strikingly 
enough, this group, the Arachnida, is the very one to which the 
scorpions belong. 
If for any cause the mouth o in Fig. 96 were to be closed, then 
the olfactory tube (o/f. pass.) might still remain, owing to its impor- 
tance as the organ of smell, and the olfactory tube would terminate 
blindly at the very spot where the corresponding tube does terminate 
in the vertebrate, according to the theory put forward in this book. 
THe OLFACTORY TUBE OF AMMOCCTES. 
In all cases where there is similarity of topographical position 
in the organs of the vertebrate and arthropod we may expect also to 
find similarity of structure. At first sight it would appear as though 
such similarity fails us here, for a cross-section of the olfactory tube 
in Petromyzon represents an elaborate organ such as is shown in Fig. 
98, very different in appearance to the section across the olfactory 
passage of a young Thelyphonus given in Fig. 97. 
Q 
