122 JOURNAL OF COMPARATIVE NEUROLOGY. 
free nerve endings in the skin without specialized sense organs 
and may be regarded as in all probability the most primitive 
type of sensory ending. 
(2) Neuromasts. This system of sense organs includes 
_the lateral canal organs, pit organs, ampullae and all other 
specialized organs associated with the lateral line canals and in- 
nervated by lateralis nerves, together with the sense organs of 
the internal ear of like phylogenetic origin and innervation. 
These organs clearly belong to asingle system and the evidence 
thus far accummulated favors the inference that the system asa 
whole has been derived phylogenetically from the general cuta- 
neous system of nerves. This is not the place to give the de- 
tailed proof of this, but the trend of the current argument may 
be summarized as follows under two heads: 
(a) the sense organs themselves are characterized in their 
adult and highly functional condition by the presence of hair 
cells differentiated from indifferent supporting cells and extend- 
ing only part way through the epithelium of the sense organ. 
These hair cells may not occur in the earliest embryological 
stages of development of the organs, nor in states of functional 
and structural degradation. The former point accounts for 
the statements of several authors to the effect that the lateral 
line organs pass through a developmental stage which is similar 
to the adult structure of the terminal buds, and the latter point 
for the confusion which has arisen in the minds of many 
authors, giving rise to the belief that there is no clearly marked 
structural difference between neuromasts of the lateral line 
series and the terminal buds. Nevertheless, the distinction 
drawn by ScuuLzE and MerKEL between the neuromasts and 
terminal buds, the former possessing the hair cells and the 
latter not, stands as an essential criterion in all cases where the 
organs are well developed. Now these hair cells resemble 
somewhat in structure and probably also in mode of function 
the tactile hairs of certain aquatic invertebrates and in fact are 
probably phylogetically derived from them. ParKER (’03) has 
shown clearly the probable line of differentiation of these 
structures from tactile organs for the perception of mass move- 
