THE CEANIAL NERVES AND LATERAL SENSE ORGANS OF FLSHES. 129 



so-called "lateral line ossicles." The anterior extremity of each "lateral line ossicle" is 

 2:)lacecl just posterior to the internal opening of a dermal tu1:)ule, so that if we regard the 

 lateral canal as composed of segments defined by the internal openings of the dermal 

 tubules and the intermediate occurrence of sense organs, then the lateral line ossicles will 

 lie at the anterior extremities of these segments. Each cylinder is imperfect externally, 

 the space being obliterated by the periosteal membrane. The length of one taken from 

 about the region of the pectoi-al fin was 7 mm. Anteriorly they increase in size, the first, 

 just posterior to the post-temporal, being an irregular imperfect tube 11 mm. long. 



E. Gadus MoitiiHUA a:nd G. vibess compared. 



The differences between these two species are few but decisive. The sense and pit 

 organs of both are the same {roughly as regards the pit-organs) both in number and 

 position. The only variations I could find were in the dermal tubules. These will be 

 at once noticed if a comparison be made between figs. 1 and 2, PLs. 21 & 23 (representing 

 G. morrlma and G. virens respectively), and are as follows :— 



1. The ninth dermal tubule on the infra-orbital canal of G. morrlma is absent in 

 G. virens. This, as far as I am aAvare, is a perfectly constant difference. I have 

 never failed to find it in G. morrlma, nor have I ever seen it in G. virens. 



2. In G. virens the hyomandibular canal opens anteriorly on to the surface, but does 

 not do so in G. morrlma. This, I believe, is another constant variation. 



3. In G. morrlma the third dermal tubule of the lateral canal is of variable occurrence. 

 In the fish from which fig. 1 was drawn it was present on the left side, but absent 

 on the right. It will be noted that the nares also were abnormal in that fish on the 

 left side, being fused there, but quite normal on the right side. In G. virens the 

 fourth dermal tubule of the lateral canal of G. morrlma is, I have found, invariably 

 absent, nor have I ever seen any fusion on the part of the anterior and posterior 

 nares. 



E. The Sense Organs on the Lateral line Canals. (Plates 22 & 23.) 



These have been carefully worked out, as well as the number and position of the 

 dermal tubules verified, from sei'ial sections of G. virens. This is the only method which 

 ensures perfect accuracy in this connection. The sense-organs themselves not being 

 obvious to the naked eye, the dissector has to rely on finding the nerve twigs supplying 

 them. If he should chance to overlook one or more of these twigs, as often happens, 

 the corresponding sense organs are, for the time at least, also overlooked, and this may 

 easily nullify the results of the investigation. I accordingly paid a visit to St. Andrews 

 and collected a large number of young " Green Cod" {G. virens), many of which have 

 since been cut into serial sections. The heads of the living animals were cut off behind 

 the pectoral fin and fixed in Hermann's platinum acetic osmic mixture. The sections 

 (10 ticks thick, Cambridge rocking microtome) were stained on the slide with Mann's 

 methyl-blue — eosin* — a most admirable method, giving perfect pictures even of the brain 



* Jouru. Anat. & Phys. vol. xsis. (N. S. vol. ix.), p. 100 (1894). 



