644 
DR. T. WILLIAMS ON THE BLOOD PROPER AND 
exprirne, par la marge du maiiteau, mais il sort de I’etroit espace situee eiitre cette 
marge et la coquille ; 6. Dans une contraction extreme, le sang pent exuder par 
toutes les parties du corps.” From the language of these propositional corollaries, it 
is certain that M. Moquin-Sandon scarcely knows what is, or what is not the hlood 
of the Planorbidse. Such uncertainty illustrates the sources of those numerous errors 
which render the historical literature of this subject really of little value. It is of 
the highest importance to the progress of knowledge that the same JLuid should be 
examined by all the observers engaged in the same pursuit. Independent observa- 
tions under such circumstances become conducive to the development of truth*. 
In the Mollusk there is but one system of fluids. It unites in itself the separate 
characteristics of the blood-proper system and the chylaqueous. In the mechanical 
character of the circulating system this union of opposite qualities is discernible. In 
the histology of the fluids it is still more so. In the transition through the INIolluscan 
route, between the Annelida and the Vertebrata, the blood of the Mollusks exhibits 
the mean of these two extreme constituents. Unlike the blood of the Annelid, it is 
the seat of floating corpuscles, and different from that of the animal ; these cor- 
puscles are not organized with regularity of plan. They exhibit more constancy of 
structure than the morphous elements of the chylaqueous fluid, less so than those of 
the blood of the vertebrated animal. The blood of the Mollusk is indeed in every 
physiological property intermediate between that of the vertebrate animal and the 
chylaqueous fluid of the Annelid. If perhaps the blood-proper of the Annelid were 
mixed with the chylaqueous fluid, the product would represent the Molluscan blood. 
Why in the one case these constituents should be held permanently separated, and 
in the other blended into one fluid, it is not easy to explain. 
In the Molluscan scale a considerable interval separates the tunicated orders from 
the Cephalopod. This interval of separation, so marked in the solids, is scarcely 
recognisable in the fluids. The blood-corpuscle of the Myriapod is far less distin- 
guishable from that of the Arachnid than the Scolopendra is from the Spider. To the 
former case this is an illustrative parallel ; so much more intimate is the affinity which 
pervades the fluids, than that which links together the systems of the solid organs in 
the animal series. 
* Preliminarily to the investigations related in the text, I am desirous in this place to state with clearness, 
the mode which I have adopted in procuring the fluid intended for examination in the Mollusca ; a class in 
which it is far more difiicult to isolate the nutritious fluids than in any other. In every instance an eye con- 
versant with these especial objects is required. The cells of the solid structures, when loosely floating in a 
fluid, may easily deceive an inexperienced observer. The blood-corpuscles in every species should first be 
unquestionably identified, by seeing them moving in the blood-channels. When, in the larger species, the 
heart is a conspicuous body and admits of ready separation from all surrounding structures, the blood may be 
drawn directly from this source. In many of the dry land species, as the Helix family, it suflices to lay open 
the mantle and expose the areola of the visceral cavity. The fluid escaping under such circumstances is true- 
blood. The smaller Mollusca must, however, with infinite patience, be submitted to microscopic examination ; 
the observer must stedfastly gaze until the soft parts are protruded beyond the limits of the shell, which in 
nearly all cases is too obscure and impenetrable to light to enable the eye to read the included living fluids. 
