The vertebrate olfactory apparatus commences as a single median tube which terminates dorsally in the lamprey, and is supplied by the two olfactory nerves which arise from the supra-infundibular portion of the brain. It is a long, tapering tube which passes ventrally and terminates blindly at the infundibulum in Ammocoetes. The dorsal position of the nasal opening is not the original one, but is brought about by the growth of the upper lip. The nasal tube originally opened ventrally, and was at that period of development known as the tube of the hypophysis. The evidence of Ammocoetes thus goes to show that the olfactory apparatus started as an olfactory tube on the ventral side of the animal, which led directly up to, and probably into, the oesophagus of the original alimentary canal of the palæostracan ancestor. Strikingly enough, although in the crustaceans the first pair of antennæ form the olfactory organs, no such free antennæ are found in the arachnids, but they have amalgamated to form a tube or olfactory passage, which leads directly into the mouth and oesophagus of the animal. This olfactory passage is very conspicuous in all members of the scorpion group, and, like the olfactory tube of the vertebrate, is innervated by a pair of nerves, which resemble those supplying the first pair of antennæ in crustaceans as to their origin from the supra-oesophageal ganglia. This nasal passage, or tube of the hypophysis, corresponds in structure and in position most closely with the olfactory tube of the scorpion group, the only difference being that in the latter case it opens directly into the oesophagus, while in the former, owing to the closure of the old mouth, it cannot open into the infundibulum. The evidence of the olfactory apparatus, combined with that of the optic apparatus, is most interesting, for, whereas the former points indubitably to an ancestor having scorpion-like affinities, the structure of the lateral eyes points distinctly to crustacean, as well as arachnid, affinities. Taking the two together the evidence is extraordinarily strong that the vertebrate arose from a member of the palæostracan group with marked scorpion-like affinities. {233}CHAPTER VII _THE PROSOMATIC SEGMENTS OF LIMULUS AND ITS ALLIES_ Comparison of the trigeminal with the prosomatic region.--The prosomatic appendages of the Gigantostraca.--Their number and nature.--Endognaths and ectognath.--The metastoma.--The coxal glands.--Prosomatic region of Eurypterus compared with that of Ammocoetes.--Prosomatic segmentation shown by muscular markings on carapace.--Evidence of coelomic cavities in Limulus.--Summary.