This position of the nasal tube suggests that it may originally have opened into the tube of the central nervous system by way of the infundibular tube. This suggestion is greatly enhanced in value by the fact that in the larval Amphioxus the tube of the central nervous system is open to the exterior, its opening being known as the anterior neuropore, and this anterior neuropore is situated at the base of a pit, known as the olfactory pit because it is supposed to represent the olfactory organ of other fishes. Following the same lines of argument as in previous chapters, this suggestion indicates that the special olfactory organs of the invertebrate ancestor of the vertebrates consisted of a single median olfactory tube or passage, which led directly into the oesophagus and was innervated, though single and median, by a pair of olfactory nerves which arose from the supra-oesophageal ganglia. Let us see what is the nature of the olfactory organs among arthropods, and whether such a suggestion possesses any probability. THE OLFACTORY ORGANS OF THE SCORPION GROUP. At first sight the answer appears to be distinctly adverse, for it is well known that in all the Insecta, Crustacea, and the large majority of Arthropoda, the first pair of antennæ, often called the antennules, are olfactory in function, and these are free-moving, bilaterally {221}situated, independent appendages. Still, even here there is the striking fact that the nerves of these olfactory organs always arise from the supra-oesophageal ganglia, although those to the second pair of antennæ arise from the infra-oesophageal ganglia, just as the olfactory nerves of the vertebrate arise from the supra-infundibular brain-mass. Not only is there this similarity of position, but also a similarity of structure in the olfactive lobes of the brain itself of so striking a character as to cause Bellonci to sum up his investigations as follows:-- "The structure and connections of the olfactive lobes present the same fundamental plan in the higher arthropods and in the vertebrates. In the one, as in the other, the olfactory fibres form, with the connecting fibres of the olfactory lobes, a fine meshwork, which, consisting as it does of separate groups, may each one be called an olfactory glomerulus." He attributes this remarkable resemblance to a physiological necessity that similarity of function necessitates similarity of structure, for he considers it out of the question to suppose any near relationship between arthropods and vertebrates. Truly an interesting remark, with the one fallacy that relationship is out of the question.