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 Tig. 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 (bat.), w T ell 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 antennae 

 of Sphasroma. 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 autennae have been so modi- 

 fied as to form an olfactory tube or passage, which leads directly 

 into the mouth and so to the oesophagus 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 in Fig. 96 were to be closed, then 

 the olfactory tube (olf. i^ciss.) 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 Ammoccetes. 



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 



