648 
DR. T. WILLIAMS ON THE BLOOD-PROPER AND 
served carefully in an unmutilated specimen while yet living, the real blood may be 
seen rolling into the axes of the tentacles, a perfectly colourless, corpusculated fluid. 
No other mode of examination is exempt from fallacy. The corpuscles are spherical 
granular cells, furnished with a nucleus, which, from its central situation, is com- 
monly indetectable. The elliptical and oat-shaped forms are never seen ; other im- 
mature cells may be observed, but the real blood-corpuscle of Planorbis corneus 
(figs. 78 and 79) is a round granular cell of extreme delicacy, colourless and pellucid. 
In the Helix and Limax families, from the large size of the specimens, and from the 
conspicuous milk-white colour of the blood-vessels, no difficulty obstructs the pro- 
cess of observing the blood. It is not much to be wondered at that every anatomist 
during the last half-century, from Cuvier to Quatrefages, have erroneously sup- 
posed the blood to be of a milk-white colour, because the vessels are so. The coats of 
the vessels are pure white, like milk ; but the blood itself is almost colourless. The 
vessels derive their colour from the presence of a layer oi adipose tissue in theu' coats. 
Why this excentric structure should exist it is not easy to explain ; but it is so. The 
readiness with which the fat-cells escape from their areolae, renders the separate ob- 
servation of the blood-corpuscles very difficult. The real blood-corpuscles are quite 
different from the fat-cells ; they are spherical granular bodies. They bear an obvious 
analogy to those of the lower molluscan groups, and yet they are different. They 
appear to consist of Jii'mer substance. The molecules filling the interval between the 
nucleus and the involucrum refract the light more abruptly. They are mingled in 
almost equal proportion with minute oval cells destitute of nucleus and without 
granules (figs. 76 and 77)- The spacious areolse which surround the intestinal canal 
are filled with true blood. When the tegumentary mantle is opened it escapes in 
considerable volume. It determines the real character of the blood in this highly 
organized family of Gasteropods. It does not partake of the milky colour of the 
arteries, as stated by Cuvier. It is pellucid, a little less so than distilled water. It 
possesses, in the highest degree, the property of coagulating. In this respect it 
strikingly differs from the chylaqueous fluid of the Annelida, which was remarked 
to be gifted in a minimum degree with the clot-forming faculty. Physiology will 
hereafter inevitably prove, that between the coagulating property and the structure 
and number of the floating cells, there exists a relation of proportion which is yet 
unresolved. The blood-corpuscles of the Whelk and Limpet (figs. 73 , 74 , 75 ) fall 
under the description now given. The Cephalopods constitute the climax of the 
molluscan series : this observation is alike true of the solids and the fluids. Like 
that of other Mollusks, bluish and colourless, the blood of the Cephalopods is rich 
in floating cells ; of more determinate and elaborate formation, however, than those 
of other Mollusks. They present far more striking uniformity in size and form than 
anything observed among the inferior molluscan families. In this fact they exhibit 
a near approximation to the vertebrate type of blood-corpuscle. These are signs of 
superior organization. They are provided always with a nucleus, situated for the 
