CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 
649 
most part centrally, but sometimes peripherally. The space between the nucleus and 
involucrum is filled with a light bluish fluid, thickly impregnated with point-like 
molecules, and here and there a larger oil-cell. Another variety of cell, wholly 
destitute of contents, may be remarked (fig. 80 ) ; these latter are probably the germ 
state of the former. Between these intermediate forms may be observed. The 
mature cells preserve a striking regularity of size and structure ; they are invariably 
capsulated, but this capsule is very thin. 
Multiplied observations will enable a future generaliser to establish, for the con- 
figuration and structure of the blood-corpuscles of the several leading orders of 
Mollusks respectively, a certain and definitive law. The blood-cells of the Bryozoa 
will have their generic characters, those of the Tunicata theirs, and the Conchifera, 
the Gasteropoda, Pteropoda and Cephalopoda severally theirs. 
Between them all there will be found some feature in common. The real law 
which presides over the conformation and structure of the corpuscular elements of 
the living fluids remains yet to be discovered. In what possible manner a mere mo- 
dification of figure can influence the agency of these free cells, the physiologist at 
present cannot conjecture. But why should greater mystery attach to the shape of a 
blood-corpuscle than to that of the body of the animal itself? The zoophytic, medusan, 
echinodermal, articulate and molluscan blood-corpuscles are only correlates severally 
of the varied organisms whieh belong to the links enumerated of the invertebrate 
series. Though destitute of colour, the floating cells of the Cephalopods resemble 
most nearly those of the blood of the Vertebrata. Thus, I trust, has been shown 
in the fluids as well as in the solids of the organisms constituting the zoological 
series, a vital and structural graduation. To the future progress of physiological 
science the clear apprehension of this truth is most important. 
Recapitulatory statement . — I have now, I trust, shown by the force of a large mass 
of evidence, that the circulating fluids in the Invertehrata occur under three distinct 
classes, distinguished from each other by prominent and unquestionable differential 
characteristics ; that the lower and lower we descend in the invertebrate scale, the 
less and less organized, the more and more like lifeless salt water the nutrient fluids 
become ; that the fluids (especially their solid elements) of the body in degrees of 
organization progress pari passu with the solids ; that classified on the basis of the 
evidence afforded by the fiuids, the articulated series really begins at the Echinoder- 
mata, and ends with the Arachnida, for throughout this series an uninterrupted line 
of continuous affinities may be clearly and indubitably traced. In every class in this 
series, either temporarily or permanently, two fiuids are provided for the nutrition of 
the organism. As the Rotifera, judged by this rule, have only a single system of 
nutrient fluids like the Mollusca, it is evident that they cannot form a link in this 
beautiful chain ; without them it is continuous, with them it is broken. The molluscan 
chain diverges from the former at the Acalephoe ; it is traceable uninterruptedly to 
the highest Cephalopod. In all the classes of which this subkingdom is constituted 
