30 BRITISH MARINE TESTACEOUS MOLLUSCA: 
the strange appearances he observed were the result of the 
contraction of the muscular fillets of the vena cava rupturing 
the extremely thin and almost invisible films of the interspaces 
of the vein, in consequence of the shock the animal received 
from sudden death, by spirit, boiling-water, or any other mode 
of asphyxia. 
I have already stated, in the observations on the Lamelli- 
branchiata, that some malacologists are of opinion, that in the 
Conchifera all the blood conveyed by arteries to the system is 
not carried back to the branchie for aération by a direct 
vascular apparatus, and that a portion of it is again passed 
into the system either through the auricle and heart or other- 
wise, without having undergone the respiratory process of 
purification. But the moderns have gone far beyond these 
venous irregularities, and assert that in every molluscum, from 
the Tunicata to the Cephalopoda, though the blood enters the 
system by arteries, a part of it reverts to the respiratory 
centre, by lacune, cavities, canals, hollows, and fissures, not 
by veins and walled tubes, which exist in the tissues of the 
flesh of the animal; and that the blood, by meanderings, exu- 
dations, and filtermgs through these labyrmthine sinuosities 
in its passage for respiratory aération, is mixed with the abdo- 
minal and alimentary cavities and their fluids, from whence 
they are collected by what are called branchio-cardiac vessels, 
and transmitted to the heart. 
With respect to this process I doubt it altogether, and make 
the same observation on it as on the Aplysia case above, that 
the excessively fine vascular membranes are at thei larger 
terminations attenuated and often destroyed by contraction 
and lost by collapse, and become in the animal killed by 
violence so amalgamated with the tissues as to be invisible, 
and thus their sites have the appearance of lacune, &c. I 
believe this to be the true solution of the apparent absence of 
venous ducts, and that, though of extreme tenuity, they exist 
in the live animal. 
Nature always acts with order and consistency, and it is 
difficult to suppose she would so far depart from these attri- 
butes, after having constituted a particular set of vessels to 
