Lateral line organs.--Function of this group of organs.--Poriferous sense-organs on the appendages in Limulus.--Branchial sense-organs.--Prosomatic sense organs.--Flabellum.--Its structure and position.--Sense-organs of mandibles.--Auditory organs of insects and arachnids.--Poriferous chordotonal organs.--Balancers of Diptera.--Resemblance to organs of flabellum.--Racquet-organs of Galeodes.--Pectens of scorpions.--Large size of nerve to all these special sense-organs.--Origin of parachordals and auditory capsule.--Reason why VIIth nerve passes in and out of capsule.--Evidence of Ammocoetes.--Intrusion of glandular mass round brain into auditory capsule.--Intrusion of generative and hepatic mass round brain into base of flabellum.--Summary. When speaking of the tripartite arrangement of the cranial nerves, an arrangement which gave the clue to the meaning of the cranial segments, I spoke of the trigeminal as supplying the sensory nerves to the skin in the head-region, and I compared this dorsal system of afferent nerves to the system of epimeral nerves in Limulus which supply the prosomatic and mesosomatic carapaces of Limulus with sensory fibres. I compared the ventral system of eye-muscle nerves with the system of nerves supplying the segmental dorso-ventral somatic muscles of the prosomatic region, and I compared the lateral system of mixed nerves with the nerves supplying the prosomatic and mesosomatic appendages of Limulus. I compared, also, the optic nerves and the olfactory nerves with the corresponding nerves in the same invertebrate group. My readers will see at once that one well-marked group of nerves--the auditory and lateral line system--has been entirely omitted up to the present, it has not even been mentioned in the scheme of the cranial segments; I have purposely reserved its consideration until now, because the organs these nerves supply, though situated in the skin, are of such a special character {356}as to form a category by themselves. These nerves cannot be classed among the afferent nerves of the skin any more than the nerves of the optic and olfactory apparatus; they require separate consideration. A very extensive literature has grown up on the subject of this system of lateral line sense-organs and their innervation, the outcome of which is decisively in favour of this system being classed with the sense-organs supplied by the auditory nerve, so that in endeavouring to understand the position of the auditory nerve, we must always bear in mind that any theory as to its origin must apply to the system of lateral line nerves as well.