I give in Fig. [96] a sagittal median section through the head-end of the animal, which shows clearly the nature of Croneberg's conception. At the front end of the body is seen the median eye (ce.), o is the mouth, Ph. the pharynx, œs. the narrow œsophagus, compressed between the supra-œsophageal (supr. œs.) and infra-œsophageal (infr. œs.) brain mass, which opens into the large alimentary canal (Al.); Olf. pass. is the olfactory passage to the mouth, lined with thick-set, very fine hairs, which spring from the hypostome (Hyp.) as well as from the large conspicuous camerostome (Cam.), which limits this tube anteriorly. The space between the camerostome and the median eye is filled up by the massive cheliceræ, which are not shown in this section, as they begin to appear in the 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 (bat.), 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 antennæ of Sphæroma. Finally, we have the observation of Blanchard quoted by Huxley, to the effect that this camerostome is innervated by nerves from the supra-œsophageal 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-œsophageal ganglia.
These facts demonstrate with wonderful clearness that in one group of the Arthropoda the olfactory antennæ have been so modified as to form an olfactory tube or passage, which leads directly into the mouth and so to the œsophagus 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 (olf. pass.) might still remain, owing to its importance 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 Ammocœtes.
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].
Fig. 97.—Transverse Section through the Olfactory Passage of a Young Thelyphonus.
1 and 2, sections of first and second appendages.
