The Origin of the Lung of AinpnUaria. 101 



tends along the right boundary of the lung, from which it is incompletely 

 separated by a fold or ridge on the body (plate 5, fig. 15; also plate 57, 

 fig. 6, Atlas of the Mollnsca of the Voyage de 1' Astrolabe). Anteriorly 

 the gill diverges from the lung and ends, together with it, on the extreme 

 left, dorsal to the heart, into the auricle, of which the veins of both organs 

 enter through a common trunk. 



The earliest stage in the development of the organs of respiration is 

 shown, in surface view, in plate i, figure 7, at rcsp. rnd.; and, in section, in 

 plate 2, figure 9. It makes its appearance as two parallel ridges, g. r and o. r, 

 separated by a furrow, /. r. As the section shows, the ridges arise as thick- 

 enings of the epithelium of the mantle. One of them, g. r, is the rudiment 

 of the gill, and the other (o. r) the rudiment of the osphradium; while the 

 furrow, /. r, is the first trace of the lung, which is ciliated. The substance 

 of the mantle, between the inner layer of epithelium which gives rise to 

 the organs of respiration and the outer layer which forms the shell -gland, 

 .?, is looselv filled with mesoderm, which is derived from the ectodermal 



o * r 



epithelium by migration. At the stage that is shown in the figure the meso- 

 derm is so arranged as to leave an unoccupied space, the pulmonary sinus, />. s. 



A section of a stage that is somewhat older is shown in figure 10. 

 Comparison with figure 9 shows that the gill-filament, g. r., grows through 

 the thickening and multiplication of the epithelial cells together with the 

 multiplication of the underlying mesoderm. In the region of the lung, /. r., 

 the thickening and multiplication of the epithelium are of such character 

 that they lead to an infolding instead of an outpushing, as is illustrated 

 by the figure. An older stage, with traces of three gill-filaments, is shown 

 in plate 3, figure n. This is a reconstruction from a series of oblique 

 sagittal sections of an embryo about 1.2 mm. long, with a shell that is 

 nearly hemispherical, sli. 



The lung, /. r., is now deeply infolded and there are three gill-folds, r.g. f. 

 The pulmonary sinus, p. s, is now a well-defined chamber under the respira- 

 tory organs, and it now opens into the auricle, au, by an aperture that is 

 not shown in the figure. The gill-filaments are filled with mesoderm in 

 which there is as yet no indications of a cavity. The mantle, mantle- 

 chamber, and respiratory organs, at this stage, are shown, more enlarged, 

 in plate 4, figure 12. The gill-filaments are filled with a compact mass of 

 mesoderm, in the plane of the section, although this is looser to the right 

 of the section and the interspaces communicate with the pulmonary sinus, 

 p. s. The lung, /. r., bends to the left as far as the osphradium, which is 

 now nearly round. 



The chamber of the mantle of an older ambryo, 1.4 mm. long, is shown 

 in plate 5, figure 14, in order to illustrate the relation of the respiratory 

 organs to other structures ; while these organs are shown, more enlarged, 

 in plate 4, figure 13, to illustrate the details. They are from an embryo 



