152 THE AMERICAN NATURALIST. [Vor. XXXIV. 
cells absorb some substances with avidity, but are totally indiffer- 
ent to others, 
Among the variable types of excretory cells two appear to be con- 
stant; the first absorbs indigo-carmine and refuses ammonium-car- 
minate, while the second precisely reverses this action. Rarely 
excretory cells do both, but even then the one more freely than the 
other. These two types are associated with voluminous organs. 
The indigo kidneys produce urea, uric acid, and urates, while in car- 
minate kidneys, thus far known, none of these substances are formed, 
though some non-indigo excretory cells contain urates. A tabular 
view of such organs for various groups of animals is reproduced 
on the opposite page, together with the products of each organ so far 
as known. 
After discussion of the special technique employed, the author 
lists the various names by which the connective-tissue cells of mol- 
lusks are designated by different investigators, and distinguishes at 
least two types of such cells: (a) Reserve cells enclosing glycogen, 
and (7) excretory cells. In the terrestrial pulmonate gastropods 
the two functions are associated in a single cell, as in the liver cells 
of vertebrates. 
In two groups of mollusks the nephridia, instead of being lined 
throughout their entire extent by a single type of excretory cell, pre- 
sent noteworthy differences: in Amphineura the reno-pericardial 
ducts of acid reaction eliminate actively carminate and litmus, while 
the rest of the nephridium, formed of different cells, and with alkaline 
reaction, eliminates indigo, In prosobranch gastropods the nephridia 
present a series of anatomical and physiological differentiations : 
Patella has two nephridia, very different in size but both eliminating 
equally indigo; in Trochus and Haliotus the larger right nephridium 
absorbs indigo alone; the left nephridium, very different in structure 
from the other, becomes faintly colored by carminate; finally, in 
monotocardic prosobranchs the single nephridium possesses two 
sorts of cells. The most numerous, non-ciliate, eliminate indigo; the 
others, ciliated, eliminate only carminate — the single nephridium 
being thus a physiological equivalent of the two nephridia in the 
Diotocardia (Trochus, etc.). 
In the Amphineura, Solenoconcha, and Gastropoda there are, in the 
connective tissue, numbers of cells acid in reaction, of which the vacu- 
oles actively absorb carminate and litmus. These scattered cells cor- 
respond physiologically to the pericardial glands of lamellibranchs and 
to the branchial heart of cephalopods which have the same power. 
