M. Koidzumt 
251 
a matrix of special protoplasm—which I think to be the same as that of the 
corbule—in which the granules are embedded. The columnar structure is 
commonly thick and distinct at the anterior part and becomes gradually 
indistinct towards the nucleus. In some individuals the columnar structure 
can be traced clearly to the anterior surface of the nucleus, but appears in 
some others to end before reaching it. The endoplasm in front of and behind 
the corbule is not continuous. In the posterior part it is less dense and 
contains various kinds of food debris. 
In the American species Leidy distinguished the “head endosarc” from 
the “body endosarc/’ but the existence of a membranous structure (corbule) 
at the boundary was overlooked by him. Porter accurately described a 
“coarsely granular protoplasmic partition,” but he also did not recognize 
the existence of a membrane. Grassi distinguished the corbule (“cestello”) 
in the Italian. form, and described it as consisting of numerous rodlets 
(“bastoncelli ”). As described above, the corbule shows no such fibrous or 
rodded structure in my organisms. The zonal arrangement of thin and dense 
areas of granules, observed very often in my organisms, does not suggest, 
to me, the existence of fibres. I conjecture the zonal appearance to be due 
to thickenings of the substance of the membrane, or a zonal arrangement of 
the granules, in consequence of contraction of the body or such other causes. 
Grassi conjectured the membrane to be continuous with the inner layer of 
the body wall, but I do not think this is so; and I am of the opinion that 
a layer of the same substance as the membrane may exist under the wall of 
the bell, keeping the corbule in connexion with the axial core of the nipple. 
I believe also that the columnar mass hanging from the base of the nipple 
to the nucleus, and the contents of the axial core, are composed of the same 
substance. A comparative study of similar structures in allied forms seems 
to afford us some basis for such conjectures. Fran 9 a described the corbule 
as an ellipsoidal sac hanging from the base of the nipple. I think his state¬ 
ment may be true, but undoubtedly the thick body wall of the bell, over¬ 
looked by him, and the upper half of his “cestello,” is nothing but the basal 
line of the wall of the bell. 
According to Kofoid and Swezy (1919) no structure corresponding to the 
corbule is noticeable in T. campanula, and there is a complicated system of 
fibres and myonemes, called by them the “neuromotor system.” They 
describe the neuromotor system as consisting of oblique fibres, longitudinal 
ridges with the basal granules of the flagella, longitudinal myonemes, and 
transverse myonemes; the first three being continuations of the axial core of 
the nipple (“centroblepharoplast ” in their terminology). As far as my ex¬ 
perience goes, there are no such fibres or myonemes recognizable in my 
organisms. As for the corbule in T. campanula , I conjecture that a similar 
but less differentiated structure may be present. 
The nucleus is oval (10-1 8/jl x 8—12/x), and is provided with a thin but 
rigid membrane. It consists of a structureless ground substance and a variable 
