No. 3-] SOME NEW FACTS ABOUT THE HIRUDINEA. 591 
resented as having made the supremely ridiculous mistake of 
taking the wart-like protuberances found on many species of 
Clepsine for sense-organs. In C. marginata —to cite one more 
case —I reported four anal ganglia. My statement was made 
with reference to a single species, and is perfectly accurate ; 
but Mr. A. charges me with an error, because, forsooth, my 
statement is not true for a// leaches. This is the style of criti- 
cism indulged in throughout this preliminary monograph. What 
I have called visual cells are put down, er cathedra, for “ fat- 
cells,” “gland-cells,” etc. Charity and necessity alike commend 
us in taking immediate leave of such oracular wisdom. It is to 
be hoped that before that impending final monograph is launched, 
our author will have discovered the unregenerate source of his 
present afflatus. 
15. The key to the analytical study of the external form is to 
be found in the metameric disposition of the sense-organs. Of 
course internal structure is to be taken into account, and a fair 
critic would hardly have fallen into the mistake of supposing 
that I had neglected this side of the subject. I have had con- 
siderable opportunity to learn the practical value of this key for 
systematic purposes, and the longer I use it the more indispen- 
sable it proves. The terminal somites are of the highest impor- 
tance for specific diagnosis, and their annular composition, which 
offers so much of theoretical interest, cannot be deciphered 
without the aid of the segmental sense-organs. What clearer 
demonstration of all this could be desired than has already been 
furnished in the case of the ten-eyed Hirudinea? I am now 
prepared to show that the same holds true of both fresh-water 
and marine Rhyucobdellide, although the application of the 
method is, as a rule, more difficult here than in the Guathob- 
dellide. The number of ventral ganglia is generally supposed 
to be the same for all the different species, genera, families, 
and so on for the entire class. While we may be sure that the 
number of somites represented in the rings does not exceed the 
number of ganglia, is it not perfectly clear that the latter can- 
not serve as a guide in determining the aznular limits of somi- 
tes, particularly at the ends of the body? Then it must be 
remembered that the number of ganglia in the cephalic group 
has never been satisfactorily determined, and even the number 
