BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 137 
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 
(also 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 physio- 
logical 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 brain on the other. In fact, these sense 
organs may very well be regarded as special sense organs 
of the gill-clefts or as branchial sense organs. This 
conclusion Prof. Froriep and I have independently arrived at. 
From the above and from the facts of development recorded 
