102 Papers from the Marine Biological Laboratory at Tortugas. 
with a shell that nearly half covers it, with knob-like tentacles with eyes, 
and other approximations to the structure of the adult. The gill runs 
obliquely from left to right, and its filaments are visible through the shell 
in an entire embryo. 
Figure 14 is a transverse section through the posterior half of this 
embryo. On the right, near the lower point of the mantle-chamber (near 
the top of the figure), is the rectum, rec; a cylindrical tube lined with 
columnar epithelium. Above the rectum (to the right of it in the figure) 
is a well-defined group of spindle-shaped mesoderm cells, the kidney, k. 
On the right (bottom of the figure) is the thin-walled pericardium, per. 
Above (to the right of) this is the pulmonary sinus, p.s. The gill-filaments, 
g.f, now project freely into the chamber of the mantle, m. c, the newer ones 
arising on the left (below in the figure) between the older filaments and 
the lung, /. g, which is a deep groove running forwards (downwards in the 
figure). The gills and lung are shown, more enlarged, in plate 4, figure 13. 
The gill-filaments are outfoldings of the columnar epithelium of the mantle, 
and they are loosely filled with a tissue of spindle-shaped mesoderm cells, 
between which are spaces that communicate with the pulmonary sinus, #. s. 
The lung-groove is lined with elongated ectoderm cells covered, on its 
internal surface, by a cap of mesoderm cells which arise from the ectoderm, 
as the illustration shows. 
The chamber of the mantle, with the lung, /., the gill, g. f, and the ad- 
jacent organs, are shown, at four successive stages of advanced develop- 
ment, in plate 6, figure 17; plate 6, figure 16; plate 7, figure 18, and plate 
5, figure 15; the last figure showing the respiratory organs at the time of 
hatching. In this series, figure 16 is from an embryo 1.8 mm. long. The 
shell is shown, at this stage, in plate 1, figures 5 and 6, with one complete 
turn in the spiral. 
The section shown in figure 17 is one that, if continued, would pass through 
the stomach, the liver, the operculum, and the part of the foot that is 
ventral to the operculum. The part of the section that is figured shows, 
besides the respiratory organs, the rectum, rec; the kidney, k,; the osphra- 
dium, osph, and part of the pericardium, per. The lung, /., is a deep pit, 
with an opening into the chamber of the mantle and is constricted by a 
projecting fold, r, of the respiratory region. While figure 16 is from an 
older embryo, it shows the anatomical relations that would be shown in 
another section of the stage shown in figure 17, and the two figures may 
be regarded as showing the lung, /., in two sections of a single specimen. 
In figure 16 the cavity of the lung, /,, occupies nearly the whole thickness 
of the mantle, so that its lining of epithelium is separated from the epi- 
thelium of the shell-gland only by a thin layer of mesoderm. In this 
figure the intestine, int, is shown as a fold in the wall of the stomach. 
Figure 18 is from an embryo about 2.1 mm. long, with a shell like that 
