30 BEITISH MAEINE 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 branchiae for aeration 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 lacuna, 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 filterings through these labyrinthine sinuosities 

 in its passage for respiratory aeration, 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 their 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 lacuna, &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 



