332 End-Organs of the Trigeminus and Lateralis Nerves 
the ear sense organs arose, and shows them to be fully formed and func- 
tional in the lowest vertebrates above Amphioxus. The Acustico-lateral 
system of nerves and sense organs in Bdellostoma is as a whole not better 
developed, in comparison with the fishes (e. g., Elasmobranchs), than is 
the Marsipobranch ear in comparison with the Elasmobranch ear. As 
we shall show, it stands as an intermediate stage between an earlier and 
still undescribed condition, and that found in higher forms. The facts 
relative to the Acustico-lateral system of Bdellostoma constitute another 
al 
() 
we Qe! 
a (2 
Fic. 7. Cross-section of epidermis of young Bdellostoma, 11 inches long, 
through lateral line canal. Preserved in formalin. 6b —basement membrane, 
c¢=spindle-cell neuromast at side of dermal groove, g—lateral line canal, 
m —=mucous cell, N = central spindle-cell neuromast ridge. X 333. 
and an important contribution to the growing evidence that the Marsi- 
pobranchs are not degenerate forms, but occupy a place in the direct line 
of descent of the phylum from the ancestral form. 
In Bdellostoma the canals of the lateral line system are certainly diffi- 
cult to find largely because they are exceedingly small, and the surface 
indications are so faint that even in heads preserved in chromic acid, only 
an eye that knows exactly where to look and what to expect can detect 
them in the full-grown adult. In the young hagfish not more than eleven 
or twelve inches long, the chromic acid treatment shows them much more 
plainly. 
