BRANCHIAL SENSE ORGANS IN ICHTHYOPSIDA. 113 
branches, should grow right away to the tail, and supply a very 
long series of branchial sense organs. 
In a former note! I put forward certain hypotheses con- 
cerning the posterior roots of spinal nerves to account for the 
apparently abnormal innervation by the vagus, that is, by a 
cranial nerve complex, of a region extending right to the tail. 
These hypotheses I now see reason to reject, and after a study 
of the actual facts of development in Elasmobranchs, as now 
recorded, I can only conclude that the so-called lateral line 
only differs in length and direction of growth from the other 
branchial sense organs. Its length is sufficiently accounted 
for by its containing the elements of at least four supra- 
branchial nerves, and its direction offers in itself nothing really 
remarkable, for the direction of growth of the other supra- 
brauchial branches is not always the same. Those of the fifth, 
seventh, and ciliary grow forwards ; those of the glosso-pharyn- 
geal and vagus I grow dorso-anteriorly, and that of the rest of 
the vagus grows backwards (figs. 46 and 51). 
In fact, the direction of growth of sense organs and nerves 
would seem to be determined by the usefulness or need of 
having branchial sense organs in regions of the body other 
than the region just above the gill-clefts where they primitively 
occur. 
Judging by the great variations one meets with in the 
arrangement of these branchial sense organs in Ichthyopsida 
it would seem as though different families of fishes and 
Amphibians had independently solved the matter for them- 
selves. The great morphological point to be noticed, 
and I shall lay great stress on it later, is that at first there 
is the rudiment of one branchial sense organ with its 
associated ganglion over each gill-cleft or over the 
site of a potential gill-cleft. 
With reference to the hypotheses about spinal nerves men- 
tioned above, I may here state that I see no reason now for 
assuming that true spinal nerves were ever connected with 
_ | Beard, “On Segmental Sense Organs, &c.,” ‘Zool. Anz.,’ 161, 162, 
1884. 
VOL, XXVI,—NEW SER, H 
