446 Dr. G. Rolleston and Mr. C. Robertson on the Aquiferous LI. On the Aquiferous and Oviducal System in the Lamelli-branchiate Mollusks. By George Rolleston, Esq., M.D., F.L.S., Linacre Professor of Anatomy; and C. Robertson, Esq., Demonstrator of Anatomy, Oxford*. Very different explanations have been offered of the means by which certain of the Lamelhbranchiata 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, dif-ferent as they are both in principle and in detail, admit yet of being 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 animaFs 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 effected by the agency of the blood-vascular system alone ; or, thirdly, they hold the effect in question to be due to the joint working" of these two systems of tubes. Agassizf refers the great distention observable in the foot of the Natica herosy 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 von Hessling J, who obtained the same result of in-jecting 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 Pyrula ^ speaks of the system (which on these grounds he holds to be continuous) as but one system and that a blood-vascular system, with certain orifices patent and communicating with the external medium in which the animal lives. Von Hessling holds also that the distention of the foot may be in part due to water inhaled through the organ of Bojanus, and mingled thus with the blood, as we shall presently describe. M. Langer§ holds that the organ of Bojanus is the route by which the water, upon which the change of volume in the animaPs body depends, passes into it, and that this water passes into the blood-vessels, and not into any specialized water-vascular system. * From the Philosophical Transactions, Part I. for 1862, p. 29. t Zeitschrift fiir wiss. Zoologie, Pt. 7-p. 1/6, 1855. X Perlrauscheln und ihre Perlen. Leipzic, 1859 : p. 241. § Denkschriften d. Kaiserlich. Akad. Wiss. xii. p. 55, 1856.