Quarterly journal of microscopical science . ng freely in the pallial the rectal side the extremity of each plate is free for a shortdistance. The histology and structure correspond in general with thatof Bythinella, but Bregenzer does not indicate the relationof the gill to the adjacent parts of the circulatory system. On the whole if would seem that in P. ventrosa theapical portion of the lamellae is more elongate and triangularin transverse seel ion than in other forms. It should be ANATOMY OF PALUDESTRINA 173 noted that in this form and Bythinella the cilia are notdistributed all
Quarterly journal of microscopical science . ng freely in the pallial the rectal side the extremity of each plate is free for a shortdistance. The histology and structure correspond in general with thatof Bythinella, but Bregenzer does not indicate the relationof the gill to the adjacent parts of the circulatory system. On the whole if would seem that in P. ventrosa theapical portion of the lamellae is more elongate and triangularin transverse seel ion than in other forms. It should be ANATOMY OF PALUDESTRINA 173 noted that in this form and Bythinella the cilia are notdistributed all over the lamellae as in Vitrei la but areconcentrated at the apex. Each plate shows longi-tudinal folds towards its basal part, becoming flatter wherethey join up with the efferent vessel. In Bythinella,Vitrella, and Paludestrina jenkinsi the lamellaeare flat and unfolded. This folding seems rather difficultto explain. Were it not for the fact that similar foldingoccurs in P. u 1 v a e (Henking) I would be inclined to think Text-fig. 6. ay. Gills in section, av, afferent vessel; ev, efferent vessel; pv, pul-monary vein. that it might be clue to shrinkage arising from excessivecontraction of the transverse muscles in each filament. Butin addition to the occurrence of similar folds in P. u 1 v a ethere is the fact that, if it were due to shrinkage, one wouldexpect such contraction to take effect over all the lamella,which is not the case. If this ultimately proves to be aninvariable character of these two species of Paludestrinait may very well be correlated with their brackish-water andmarine habitat. It should also be pointed u! that unlessthe text-figures (i and H in Bregenzers paper are diagram-matic the afferent vessels and blood-spaces are much smallerin P. ventrosa than in Bythinella. N 2 174 GUY C. ROBSON From the rectal sinus blood passes by rather irregular andinconstant lacunar spaces to the roof of the pallial finally into more definite and constant lacuna
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