No. 2.] THE MORPHOLOGY OF DERO VAGA. 163 
organs and the sense organs of worms, but from the present 
status of our knowledge of the subject we cannot draw any 
valuable conclusions. 
4. THE SO-CALLED “ LATERAL LINE.” 
(a) Azstorical. 
This structure has been a problematic one for twenty years, 
and I think that at last the true nature of the “lateral line”’ 
of annelids has been discovered. With this thought in mind I 
give below an extended historical account setting forth the 
various views that have been held regarding it, and have tried 
to interpret the observations of various investigators on this 
cell cord. 
To Carl Semper is given the credit of the discovery of the 
so-called “lateral line” in annelids. In his extended work on 
the relationships of segmented animals! published in 1876 he 
described and figured this structure and discussed its probable 
significance. 
He characterized it as a row of cells belonging to the side 
tract of the worm, lying between the two lateral bands of mus- 
cles and extending from the tail, where it originates from the 
ectoderm, to the front of the head, where it passes into the 
oesophageal collar. In the case of individuals undergoing fis- 
sion, the cord forms in the developing head a sensory plate 
which gives rise to a part of the brain and commissure, and as 
it seemed to him might even give rise to muscular fibres. 
He found the cell cord to exist in Chaetogaster, Nais, Tubi- 
fex, and Psammoryctes. In a species of the last genus he 
figures the connection of the cord of cells with the commissure 
near the brain.? 
Semper considers that this annelid lateral line may be com- 
pared to the lateral line of the lower vertebrates, and thinks that 
there is a resemblance between it and the nerve of the lateral- 
line system of fishes and larval amphibians, his reasons being 
1C. Semper, Die Verwandtschaftsbeziehungen der gegliederten Thiere, III, 
Strobilation und Segmentation, Arbeit. a. d. Zool.-Zoot Inst. Wiirzburg, Bd. III, 
1876. 2 Semper, /.c., Taf. XI, Fig. 3, s7. 
