THE CEANIAL NERVES AND LATEliAL SENSE ORGANS OF FISHES. 131 



G. Other Sense Organs belonging to the Lateral line System. (Plate 21.) 



Belonging undoubtedly to the lateral line system, since they are innervated by the 

 same nerves, are the pit oi'gans or sensoi'y follicles, described by Merkel, Fritsch, AUis, and 

 Ewart & Mitchell *. These in the Cod are somewhat laiimerous, and do not occur in the 

 well-defined series that they do in the Elasmobranchs and Ganoids. Their structure 

 has already been fully discussed by the authors above mentioned, so that only a few 

 words are necessary as to their distribution. As will be seen by reference to tlie figure 

 of G. morrhua, the pit organs cannot be separated into groups, being too irregularly 

 scattered. It is equally obvious, however, that most of them occur in connection with 

 the sensory canals, and this is most marked in the case of the hyomandibular canal. In 

 spite of their apparent irregularity and number, they are very constant both in number 

 and position, as will be ascertained by a careful examination of an adult G. morrlina 

 and a comparison with the figure. The pit organs are usually conspicuous on account 

 of their pigmented lips, and even where this is slight or absent a contrast in colour 

 usually betrays their existence. As is well known, the pit oi'gans are excavated papillae 

 formed by an invagination of the skin, and lodging a sense-organ innervated by a twig 

 from one of the lateral line nerves. My figure (PL 21) of these organs is the result of a 

 minute examination of the heads of several large specimens of G. morrhua. 



Contrasting with the numerous pit organs is tin; complete absence of any traces of 

 Lorenzini's amiiulUe and the associated ampuUary canals. Tliis fact is duly commented 

 on elsewhere. 



H. PiElation of the Sensory Canals to the Skull. 



That the lateral line system has no definite relations with the primitive skull is quite 

 evident even on a priori grounds. An epiblastic structvu'e, necessarily connected with 

 the skin, and actually found on the surface in Chim<era, or associated with it in many 

 forms, can only be secondarily connected with the skull. Except in the Elasmobranch 

 fishes, where the lateral canals are firmly embedded in the extensive subdermal tissue, 

 there is a tendency, almost in all fishes, to protect the sensory canals with some form of 

 calcified or rigid support. In Chimcera this takes the form of imperfect limy rings, 

 whilst in the majority of bony fishes the support ranges from a partial to an absolutely 

 perfect bony tube. There is, in fact, a strong tendency to surround the sensory canals- 

 with a complete extensive osseous deposit. 



Such being the case, it becomes necessary to tlistinguisli between lateral line bones 

 and the cranial bones sensu stricto, and to enquire into the connection between the two 

 series where such must obviously exist. We may therefore divide the bones supporting the 

 lateral line into two series : — (o) ossicles developed purely for the support of the sensory 

 canals ; [h) cranial bones with which the lateral line ossicles have become secondarily 

 connected. The excellent work which has been done by McMurricli on the osteology of 

 Aminriis catus (1883-1, 136) has made perfectly clear the bearings of the lateral line 



* See full discussion below. 

 SECONB SERIES. — ZOOLOGY, VOL. VII. 19 



