ON THE PAIRED NEPHRIDIA OF PROSOBRANOHS. 617 
prove for reasons which the reader of this paper will easily 
understand, I began to look for theright one. I thought that, 
having once found the right one, its shape and position would 
enable me to see the left one. Meeting with the same nega- 
tive results, 1 then examined Haliotis and Trochus, when I 
immediately found the left reno-pericardial duct only, but 
this with the greatest ease. I then carefully re-examined my 
preparations of Fissurella and Patella, and came to the 
results which I have fully described in this paper. I next con- 
sidered it my duty to explain the contradictory statements of 
my predecessors, and hope to have done this successfully. 
Nevertheless I will be most happy to confess that I have been 
wrong in this matter, provided I am shown an uninterrupted 
series of sections through an uninjected specimen displaying 
one or both reno-pericardiac ducts in Fissurella or Patella. 
A valid proof of the existence of both reno-pericardiac ducts 
in both these species would exactly suit my theory on the ho- 
mologies of the kidney in Monotocards. 
In my paper on Bythinia I have fully expressed the opinion 
I hold on the value of the method of sections. I have stated 
that in my humble opinion a thorough study of the entire 
animal or embryo by other methods must always precede that 
of sections. However, I maintain that sections alone can, 
under the present state of anatomical technique, give us an 
exhaustive knowledge of complicated topographical questions 
and of delicate anatomical points. The method of sections is 
most valuable when the existence of a communication between 
an organ and another organ or the outer world is to be proved. 
In this case sections are the only true test. My figures show 
that when a reno-pericardiac opening really exists it has the 
shape of a well-defined canal, generally of no inconsiderable 
length, as in Haliotis and Trochus. I have seen the same 
structure in Paludina, Bythinia and Aplysia, in Signor 
Mazzarelli’s series of sections. For these reasons I think it 
is fair that I should require to be shown similar ducts in 
Fissurella and Patella beforeI give up my own view on the 
points I have discussed in this paper. 
VOL. XXXIII, PART IV.—NEW SER. ROE 
