CHYLAQUEOUS FLUID OF INVERTEBRATE ANIMALS. 647 
distinguishing the corpusciilated fluid as it escapes, under pressure, from the little 
cavity of the organ*. 
Like that of all other Mollusks, the blood in this order is colourless, limpidly 
opalescent, and charged with corpuscles which present three main varieties : — 1st, the 
round granular cell, which is probably the mature form of the blood-corpuscle ; 
2nd, a nucleated, orbicular, pellucid cell, destitute of all other contents ; and, 3rd, 
minuter globules filled only with an opalescent fluid. In the Lamellibranchiate 
family, the blood is corpusculated on one plan. It is almost impossible to indicate 
structural differences between the blood-cells of Pholas (fig. 71) and those of Pinna 
(fig. 72), or between those of Mya (fig. 69) and those of Solen (fig. 70), or those of 
the Oyster (fig. 68) and those of the Mussel (fig. 69). The blood of the small fresh 
bivalves is less obviously corpusculated, and the corpuscles themselves are less 
impregnated with granules. Although exhibiting the limpidity of pure water, it 
coagulates into clots on escaping from the body. As yet a manifest unity of plan 
in the structure of the blood-corpuscles of the Mollusca has not been found to 
prevail in the classes examined. Nowhere has there existed one invariable ever- 
present form of corpuscles, such as obtains in the blood of the articulated animal, 
and varieties present themselves in every instance. But under this variety there 
runs a legible unity. The forms of cells, various though they be, which characterize 
the blood of the lower Mollusks now examined, are undoubtedly pervaded by a 
community of structural characters; through individual diversities there runs a 
chord of continuous union. The two remaining groups of Mollusks, the Gastero- 
pods and Cephalopods, present signs of some advance upon the former in the vital 
composition of the fluids. Preserving the type of the molluscan blood-corpuscle, they 
lose some of the irregular, aberrant forms of the cells. 
Gasteropoda . — The freshwater Gasteropods, in which the blood can be seen rolling- 
in its containing channels, become serviceable as standards of comparisons for deter- 
mining the true-blood corpuscle of other species of this family. The Planorbidae are 
readily examined for the blood in the living state. The horns and foot are hollowed 
out in the interior, into spacious axial channels, into which the blood rushes under 
the compression of muscular force. This is the real mechanism by which the arms 
of the Brachiopods are extended. My observations on the blood of the Pianorbid® 
conduct me to conclusions at diametrical variance with those of Moquin-Sandon'|'. 
The true-blood of these Mollusks is colourless, not, as maintained by naturalists, red 
and purple. I demur to his method of observation. The mantle, when the animal is 
irritated, does throw out a coloured fluid, but that is not the blood of the animal. Ob- 
* As to one unpractised in these delicate researches some difficulty may attend the method stated in the 
text, a certain view of the corpuscles of the blood may be obtained by placing a minute freshwater bivalve, such 
as a Pisidium, in a cupped glass under the microscope ; when the soft parts, such as the siphons, edge of the 
mantle and foot, are being protruded beyond the limits of the shell, the movement of the blood, as it slowly 
distends these parts, can be very clearly and perfectly observed. 
t Annales des Sciences Naturelles, Serie, 1851. 
