LATERAL SENSE ORGANS OF SQUALUS SUCKLII 285 
not as uniformly developed and is much less conspicuous than it 
is in Squalus suckli. 
The fine fibrillae which arise from the secondary zone of dis- 
tribution rise to various levels in the sensory epithelium. In 
many instances they can be traced to within a short distance of 
the outside limiting membrane (figs. 8 and 9). Varicosities of 
various sizes and shapes appear on the fibrillae at practically all 
levels and not infrequently at their distal extremities. In many 
cases the fibrillae appear to surround the bases of the hair cells 
(figs. 8 and 9), and in others, to pass out freely and separately 
between the hair cells. 
The observations set forth above corroborate the results 
obtained on Mustelus canis. Only minor differences exist in the 
structure and innervation of the sensory epithelium of the two 
species. In Squalus sucklii the sensory epithelium is less ex- 
tensive, there is a stronger suggestion of segmentation, and in 
nerve supply there is a more definite and conspicuously second- 
ary zone of distribution. 
A number of features which stand out in the embryonic and 
adult structure of the lateral canal system of Squalus and Muste- 
lus appear to me to reflect doubt on the theory that this sytem of 
sense organs has a phylogenetic relationship with the segmental 
sense organs of certain invertebrates and that the system itself 
is segmental in the sense suggested by John Beard? and W. H. 
Gaskell.4 The evidence, in part, against such a view may be 
Fig. 3 Transverse section of the sensory column, showing the peculiar condi- 
tion of two groups of hair cells (Grp.) existing side by side. Camera sketch, 
xX 650. Nf., nerve fibers of the subbasilar fiber zone. 
Fig. Longitudinal section of the lateral sensory column and the subbasilar 
fiber zone (Fb.Zn.). The secondary zone of distribution (Snd.Zn.) is also shown. 
Camera sketch. Pyridine silver tech. X 650, 3 off. Grp., group or cluster of 
hair cells; VN.M.Fb., non-medullated nerve fibers. 
Fig. 5 Transverse section of the sensory column showing large enol and 
fibrillae diverging at a large varicosity. Pyridine silver tech. X 1525, 2 off. 
Fig. 6 Transverse section of sensory epithelium showing long, fine sens: and 
varicosities. Pyridine silver. > 650, + off. 
3 See Zool. Anz., Bd. 7, 1884, p. 125 et seq., and also Bd. 8. 
4 The Origin of Vertebrates, 1908. 
