DESCRIPTION ‘OF PLATES: 
Letters of Reference. 
an. Anus. oe. CEsophagus. 
a. s. Aortic sinus. o. Operculum. 
au. Auricle. o. r. Rudimentary osphradium. 
f. Foot. osph. Osphradium. 
g. r. Rudimentary gill. part. Partition of cavity of mantle. 
r. g. f. Rudimentary gill filament. per. Pericardium. 
g. f. Gill filament. p. s. Pulmonary sinus. 
r. Fold that forms part of floor p. v. Pulmonary vein. 
of lung. r. s. Radula sac. 
h. v. Head vesicle. rec. Rectum. 
k. Kidney. resp. rud. Rudiment of organs of respira- 
l. ry. Rudimentary lung. tion. 
l. g. Lung groove. sh. Shell. 
l. Lung. sh. g. Shell gland. 
am. Mouth. p. st. Primitive stomach. 
m. c. Cavity of mantle. st. Stomach. 
m. f. Edge of shell gland. v. Velum. 
n. Nerve tissue. ven. Ventricle. 
EXPLANATION OF PLATES. 
IPEATES Tero! 7. 
All of the figures are of Ampullaria depressa Say. 
PrPATE TT: 
Fics. 1 and 2. Adult, natural size. 
Fics. 3 and 4. Enlarged view of shell at time of hatching. 
Fics. 5 and 6. Enlarged view of shell of unhatched embryo 1.8 mm. long. 
Fic. 7. Surface view of an unhatched embryo 1.2 mm. long. 
Fic. 8. Eggs attached to grass. 
PLATE 2. 
Fics. 9 and 10. Sections of the organs of respiration in their earliest stages. Figure 
10 shows a stage somewhat older than that illustrated in figure 9. 
PLATE 3. 
Fic. 11. Reconstruction from a series of oblique sagittal sections of an embryo in 
the stage shown in figure 7, plate I. 
PLATE 4. 
Fic. 12. Sagittal section of mantle-chamber and respiratory organs in an embryo 
1.2 mm. long. 
Fic. 13. Transverse section of the mantle-chamber of an embryo 1.4 mm. long. 
PLATE 5. 
Fic. 14. Transverse section of the mantle chamber of an embryo 1.4 mm. long. 
Fic. 15. Section of the respiratory organs at the time of hatching. 
PLATE 6. 
Fics. 16 and 17. Section of mantle-chamber showing lung, gill, etc. Figure 16 shows 
a stage wherein the embryo is 1.8 mm. long and is older than that shown 
in figure 17. 
PEATE 7. 
Fic. 18. Section of an unhatched embryo 2.1 mm. long. 
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