Quarterly journal of microscopical science . accepted nearly by all morphologists, thatthe ancestral form of molluscs was of bilaterally symmetricalbuild, and that the anus (A.) and mantle (M.) cavity weresituated at the aboral end of the animal. Spengel,^ Biitschli(5), Ray Lankester (21), Lang (17), and others have built uptheir theories on the asymmetry and torsion on this hypo-thesis. The existence of the bilateral symmetry furtherimplies that the ancestral form possessed paired ctenidia{Br.), osphradia, and nephridia (N.). The nephridia weretubes with a medium glandular portion having a co
Quarterly journal of microscopical science . accepted nearly by all morphologists, thatthe ancestral form of molluscs was of bilaterally symmetricalbuild, and that the anus (A.) and mantle (M.) cavity weresituated at the aboral end of the animal. Spengel,^ Biitschli(5), Ray Lankester (21), Lang (17), and others have built uptheir theories on the asymmetry and torsion on this hypo-thesis. The existence of the bilateral symmetry furtherimplies that the ancestral form possessed paired ctenidia{Br.), osphradia, and nephridia (N.). The nephridia weretubes with a medium glandular portion having a communica-tion (F.) with the pericardium (Pc), and opening at the basis Spengel, J. W., Die Geruclisorgane und das Nervensystem der Mol-lusken, Zeitsclir. f. wiss. Zooi., 1881, xxxv, pp. 333—384, Taf. xvii—xix. ON THE PAIRED NEPHRIDIA OP PROSOBRANCHS. 615 of either ctenidium at the eud of a papilla (P.)- The rectum(R.) passed through the pericardium {Pc), but ventrally fromthe heart, without perforating the ventricle ( V), opening by the. --Y Diagram of the reno-pericardial system of the ancestral form of Aorta. au. Auricles. Ma. Mantle. The other letters are ex-plained in the text. anus {A.) in the middle line, at the bottom of the mantlecavity (M). Most probably the entire anterior half of the epi-thelial lining of the pericardium, that is to say, the portionanterior to the region occupied by the heart, represented thegonad (G.). The sexual products dropped into the cavity of thepericardium passed through the funnel into the nephridium,and passed out of the nephridial duct into the mantle condition would be nearly realised by the actual Neo-menise. Very soon the portion of the pericardium, the lining ofwhich represented the epithelium of the gonad, became con-stricted off the pericardium (compare the development ofPaludina). This is the case in all other groups of Moliuscaexcept Neomenise. According to Pruvot, the gonad in Aplacophora isformed by tw
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