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II. On the Aquiferous and Omducal System in the Lamellibranchiate Mollusks. 
By George Eolleston, Esq.^ M.D., F.L.S., Linacre Professor of Anatomy ; and 
C. Eobertson, Esq.^ Lemonstratm' of Anatomy, Oxford. Communicated hy Br. Acland. 
Keceiyed August 20, — Eead November 21, 1861. 
Vert difiFerent explanations have been offered of the means by which certain of the 
Lamellibranchiata are enabled to distend their muscular foot until the fluid with which 
it is swollen up causes it to appear all but transparent. These explanations, different 
as they are both in principle and in detail, admit yet of bemg reduced under one or other 
of three heads. Either they postulate the existence of a system of tubes homologous 
with the tracheae of insects, and, like them, distinct from the animal’s blood-vessels, as 
necessary for the explanation of the great changes of volume observed to take place in 
the mollusk’s body ; or they suppose these alterations of size to be efiected by the agency 
of the blood-vascular system alone ; or, thirdly, they hold the efiect in question to be due 
to the jomt working of these two systems of tubes. 
Agassiz * refers the great distention obseiwable in the foot of the Natica heros, of the 
Pyrula carica and canaliculata, and the Acephalous Mactra solidissima, to water inhaled 
by orifices more or less numerous, of less or greater calibre, in the muscular foot : these 
orifices, and the tubes in connexion with them, he speaks of as a water-vascular system, 
but he holds that they come into more or less direct and constant communication with 
the true blood-vascular system. 
Theodor vox HESSLixof, who obtained the same result of injecting fully the blood- 
vascular system, by throwing in fluid from the glandular depression in the foot of the 
Unio margaritifera, as Agassiz did by a similar procedure with the similar depression in 
the foot of the Gasteropodous Pyrulce, speaks of the system (which on these grounds he 
holds to be continuous) as but one system, and that a blood-vascular system, with cer- 
tain orifices patent and communicating with the external medium in which the animal 
lives. Vox Hesslixg holds also that the distention of the foot may be in part due to 
water inhaled through the organ of Bo J ax us, and mmgled thus with the blood, as we 
shall presently describe. 
M. Laxger;{I holds that the organ of Bojaxus is the route by which the water, upon 
which the change of volume in the animal’s body depends, passes into it, and that this 
water passes into the blood-vessels, and not into any specialized water-vascular system. 
♦ Zeitschrift fur wiss. Zoologie. Pt. 7. p. 176, 1855. 
t Perlmuscheln und ihre Perlen. Leipzic, 1859, p. 241. 
t Denkscliriften, d. Kaiserlich. Akad. Wiss. xii. p. 55, 1856. 
