12 THE NAUTILUS. 



shallow water of the Everglades, we found great numbers of small 

 eggs attached to the stems of the reeds and grasses above the surface 

 of the water but close to it. 



The eggs were arranged in vertical rows, and were enclosed in 

 calcareous shells, resembling in these respects the eggs of terrestrial 

 pulmonate gasteropods. 



We also found in the water in great abundance the prosobranchiate 

 gasteropod Ampullaria, and when some of the older eggs were opened 

 they were found to contain young specimens of this genus. 



The Paludinidee, which are closly related to the Ampullaridx, are 

 aquatic, viviparous, and breathe by gills, and their structure indicates 

 that they are true prosobranchs, descended from and closely related 

 to the marine prosobranchs. Ampullaria has gills, is partly aquatic, 

 and seems to be a true prosobranch, so far as its general structure is 

 in question, but as it has a lung, and is able to breathe air and live 

 out of the water, and as it also lays, in the air, eggs in calcareous 

 shells, like those of the terrestrial pulmonates, the question whether 

 it is primarily a pulmonate, with secondary resemblance to the pro- 

 sobranchs or primarily a prosobranch with secondary resemblance to 

 the pulmonates, suggests itself. 



As the embryonic history of the breathing organs may be expected 

 to throw light upon this question, a quantity of the eggs were col- 

 lected and taken to the Marine Laboratory in the Dry Tortugas. 

 There the eggs were opened, the embryos removed and sketched, and 

 then hardened and preserved for embryological examination. 



On my return to Baltimore I placed the material in the hands of 

 Mr. B. McGlone, who has studied the development of the respiratory 

 organs under my supervision, and has nearly completed his work, 

 which will soon be ready for publication. He has shown that the 

 lung of Ampullaria is a member of the series of gill-filaments, and 

 that it must be regarded as a modified gill, homologous with a cteni- 

 dium, or with more than one. It is therefore an organ which has 

 been secondarily acquired, and not derived from the lung of the ter- 

 restrial pulmonates. 



Both lung and gills arise very early in the embryonic history of 

 Ampullaria, and at about the same time. In a very young embryo, 

 soon after the mantle makes its appearance, a ridge or thickening of 

 the epithelium of the inner surface of the mantle indicates the region 

 from which the gill-filaments, the lung and the osphradium are to 

 arise. The osphradium is developed from one end of this ridge, the 

 gill-filaments from the other, and between the two the ridge becomes 

 infolded into the substance of the mantle to give rise to the lung, 

 which may be regarded as a modified and invaginated gill-filament. 



The similarity between the lung of the pulmonates and that of 

 Ampullaria is therefore nothing more than a new illustration of a 

 resemblance between organs that have been acquired independently 

 under like physiological conditions. 



