THE CRANIAL NERVES AND LATERAL SENSE ORGANS OF FISHES. 205 



14. The so-called lateralis nerve of Fetromyzon is not a lateral line nerve at all, but its 

 anatomy sliows it to belong vindoubtedly to the lateralis acccssorius system. 



15. The lateral line system of Gadus is in remarkable agreement with that of Ainia, 

 except that the latter form exhibits less reduction in the number of its sense organs. 



16. The sense organs of the lateral line of Pishes and Amphibia are not lioiuologous 

 ■with the lateral sense organs of Annelids {e. g. Ca})itellidte). 



17. The sensory canals were probably represented in the ancestral Vertebrate by non- 

 segmental sujierficial sense organs. These sank below the sui-face, forming a series of 

 tubes, by the end-to-eud fusion of which the sensory canals were formed. As the sense 

 organs sank below the surface, certain lateral glandular organs were also included which 

 secreted the mucus tilling the canals. These glands have been subsequently much 

 reduced in importance, and their function now is an accessory one, l. e. to secrete a 

 substance corresponding precisely to the endolympli of the ear. Before the sensory canal 

 system extended forwards and backwards it was probably confined to the region now 

 occupied by the auditory organ. 



18. There can l)e little doubt that the primitive lateral line organs gave rise both to 

 the recent lateral system and also to the vertebrate auditory organ. It is doubtful 

 whether the semicircular canals have arisen by a concentration of primitive sensory 

 canals or by the decentral development of a primitive simple sac. The author holds to 

 the latter view, and believes that the semicircular <*.anals are homoplastic and not 

 homologous with tiie sensory canals. 



19. The author's previous view that the chorda tympani nerve of Mammals has been 

 derived from the prae-spiracular division of the facial of the Piscine ancestor of the 

 Vertebrates is confirmed. The nerve referred to as the " internal mandibular " by Allis 

 and some other authors is wrongly identified, and is really the prae-branchial or chorda 

 tympani division of the facial. 



20. A typical facial nerve of a fish has the constitution tabulated on p. 202. 



Zoological Department, 



University College, Liverpool. 



March 2ud, 1898. 



Postscri2)t. — Since writing Section H, I have examined a large number of Gad/is skulls, 

 and have no doubt now that the description does not represent the normal condition, which 

 in the vast majority of the specimens examined was that of complete fusion between the 

 dermal and cartilage pterotic elements, although the point of fusion could always be easily 

 detected. The nomenclature of these two l)ones is lience in urgent need of revision. 

 The terms post-frontal and squamosal should be reserved for the separate dermal lateral 

 line elements, and sphenotic and pterotic for the separate cartilage auditory bones. A 

 comj)ound term is also necessary for the condition in which the lateral and cartilage 

 elements have secondarily fused to form one bone. Where the lateral line bones are never 

 known to be separate from the underlying cranial bones in the adult there is no pressing 

 need in the meantime for any revision of the nomenclature. — Sept. 27, 1898. 



28* 



