PRIAPULUS AND HALICRYPTUS. 
207 
the skin, both of them having sensory hairs. He has estab- 
lished experimentally that the apparently different structures 
are one and the same thing, the latter' being simply an invagi- 
nation of the first kind. 
The most noteworthy analogy to the sensory organs of 
Priapulus and Halicryptus, however, is to be found among the 
lower Vertebrates, such as fishes and larval Amphibians. In 
order to show how closely the organs of which the so-called 
lateral line in fishes is composed of agrees with those I have 
described, I will give a few extracts from some of the more 
important works on this subject. 
Leydig 1 was the first to prove the nervous nature of these 
organs, which were generally believed to excrete mucus, and 
showed that they were sensory organs peculiar to fishes. A 
few years later he wrote an excellent and well-known treatise 2 
on sensory organs in general, and attributed to those found in 
the lateral line of fishes the function of a sixth sense unrepre- 
sented in the higher Vertebrates. 
M. Schultze, as well as F. E. Schulze, have extended the 
knowledge about these organs very considerably. It was the 
latter who first discovered the peculiar protuberant structures 
in the skin of young fishes, and indicated that they were peri- 
pheral sense organs corresponding to those of the lateral line 
found in adult fishes. Shortly after he published another 
paper 3 in which he describes their histological structure : — 
“From the epithelial cells, of which these organs are chiefly 
made up and which stand in connection with nerves, I saw a 
number of delicate stiff hairs projecting into the water similar 
to those found on the Crista acustica, only much shorter.” 
Moreover, he describes “ a slender tube, rising from the margin 
of this structure, open at the end and obliquely truncated.” 
1 Leydig, “ Ueber die Schleimkanale der Kuochenfische,” ‘ Muller’s 
Archiv,’ 1850. 
2 Leydig, “ Ueber Organe eines sechsten Sinnes,” ‘ Nova Acta Leop. Carol./ 
vol. xxxiv. 
3 F. E. Schulze, “ Ueber die Sinnesorgane der Seitenlinie bei Fischen und 
Amphibien,” ‘ Archiv f. mikrosc. Anatomie,’ vol. vi, p. 63. 
