BRANCHIAL SENSE ORGANS OP ICHTHTOPSIDA. 205 



Before the auditory involution has proceeded very far there is a 

 considerable ganglion formed, and fused -with the auditory thickening 

 (fig. 29). At this stage the whole nerve, sense organ, and ganglion 

 correspond exactly with the nerve, sense organ, and ganglion of the 

 ciliary segment (fig. 8). 



Soon the involution is carried to such an extent that the auditory 

 organ forms a sac, but it still opens on to the surface, and in Elasmo- 

 branchs does so throughout life. Even after the formation of the 

 sac, cells continue to be given off from the thickening to form the 

 ganglion (fig. 31). The later-formed semicircular canals, &c, are 

 obviously secondary complications, which have as their motive the 

 extension and perfection of the sensory surface, and which resemble 

 somewhat the formation of a supra-branchial nerve and its sense 

 organs. 



The resemblance in structure between the sensory cells of the ear 

 and those of the branchial sense organs is obvious enough, and need 

 not be dilated upon here. 



In Amphibia (Rana temporaria) the auditory organ, nerve, &c, are 

 formed just like the sense organ, nerve, &c, of the trigeminus of the 

 same animal. The nerve is split off from the epiblast, the auditory 

 thickening is developed from the deeper layer of the epiblast opposite 

 the notochord, and, as in the stage figured (fig 28), there is no 

 auditory ganglion, it is fair to assume that it is formed just as in 

 other cranial posterior nerves in Amphibia in connection with the 

 auditory thickening. 



In Elasmobranchii, &c, the auditory ganglion and nerve become so 

 fused with the facial that the nerve has usually been described as a 

 branch of the facial. "We have seen that it developes separately from 

 the facial, and even when partially fused (fig. 21), the line dividing 

 the two nerves is readily seen (cf. Marshall). 



General Considerations. 

 Morphology of the branchial sense organs. — It is pretty clear from 

 the facts recorded in the preceding pages that the so-called organs of 

 the lateral line have some physiological relationship with the gill-clefts. 

 They arise at the same time as the latter, are originally seated one 

 over each gill-cleft, and have each a ganglion of a dorsal root of a 

 cranial nerve arising with and attached to them. From the ganglion 

 nerve-fibres pass to the gill-musculature on the one hand, and to the 



