1895.] Dr Gaskell, The Origin of Vertebrates. 29 



The history of the Vlllth nerve. 



Throughout the vertebrate kingdom the auditory apparatus 

 remains constant in position, varying in structure to the extent 

 that the cochlea becomes more developed as we pass upwards, yet 

 throughout a gradual development can be traced from the simple 

 auditory capsule of the Cyclostomi to the complicated bony and 

 membranous labyrinths of man; the changes which took place owing 

 to the passage from water breathing to air breathing animals, the 

 formation of a wembrana tympani with the remarkable history of 

 the ossicles does not concern the history of the Vlllth nerve. We 

 see then that the auditory apparatus and the Vlllth nerve arises in 

 connection with a simple auditory capsule and a special sense nerve 

 which is situated between the trigeminal and vagus group of 

 nerves. Going one step further this means that an auditory 

 apparatus and nerve similar to that of Ammocoetes existed be- 

 tween the locomotor or thoracic appendages and the branchial or 

 abdominal appendages of the Eurypterus-like animal from which 

 the Ammocoetes arose, and when we turn to Limulus we find 

 exactly in the same position an organ which is the counterpart of 

 the auditory organ of Ammocoetes. For, situated at the base of 

 the last large digging appendage — between therefore the locomotor 

 appendages and the operculum — is found a large hemispherical 

 organ to which a moveable spatula-like process is attached, known 

 as the fiabellum and called by Ray Lankester an " epipodite." 

 This organ is confined to the base of this limb ; it is undoubtedly 

 a sense organ, being composed mainly of nerves, and an elaborate 

 arrangement of nerve cells in connection with innumerable fine 

 hairs, which are thickly embedded in the chitin of the upper 

 surface of the spatula. By the varying thickness of the chitin 

 these sense hairs vary with considerable regularity in length, and 

 when the animal is at rest this sensory surface projects upwards 

 and backwards into the crack between the thoracic and abdominal 

 carapaces, so that while the eyes only permit a look out forwards 

 and sidewards, and the whole animal is lying half buried in the 

 sand, any vibrations in the water around can still pass through 

 this open crevice and so reach the sensory surface of this organ. 

 The structure of this sense organ is of a somewhat similar cha- 

 racter to that of the lyriform organs described by Gaubert and 

 supposed to be auditory. 



Further, the most striking and complete proof that this sense 

 organ of Limulus is the same as the auditory capsule of Ammo- 

 coetes is found in the fact that in each case the nerve is accom- 



