100 }6©6-s Papers from the Marine Biological Laboratory ai Tortugas. 
It is surrounded by a folded ridge, mf, the margin of the shell-gland. 
In plate 3, figure 11, the shell covers the whole of the posterior end of the 
body, and the raised edges of the shell-gland are pushed far apart. The 
growth of the shell brings about changes in the position of the mantle-cham- 
ber, in that of the mantle itself, and in that of other organs, as will be 
noted later. 
From the mouth the cesophagus, oe, runs upward, within the head- 
vesicle, to the dorsal anterior region, where it opens into a huge chamber, 
the primitive stomach, pr. st., which fills most of the head-vesicle. The 
sac of the radula (1, fig. 11) arises from the posterior (ventral) border of 
the cesophagus near the mouth, and extends under the ventral surfuce of 
the primitive stomach. A pair of salivary glands that are not shown in the 
figure arises from the cesophagus, near the mouth, as a pair of pouches or 
outgrowths. The primitive stomach narrows posteriorly and gives rise to 
the intestine, which, arising on the left, bends to the right, and opens into 
the cavity of the mantle, through the anus, a, which is at first median and 
ventral, but moves upward on to the right side of the mantle as development 
advances. Near the anus there is, in young embryos, a rosette-shaped clus- 
ter of large transparent cells like those that have been mentioned on the 
foot. They persist until about the time of hatching. The buccal and 
cerebral ganglia arise as compact groups of spindle-cells below and above 
the radula, while a similar group of cells arises in the posterior part of the 
foot and becomes the pedal ganglia. 
The heart is shown in plate 3, figure 11, near the bottom of the figure; 
although the opening through which the auricle, aw., communicates with the 
ventricle, ven, does not lie in the plane of this section. The heart is inclosed 
in a spacious pericardium, per, with a thin wall of spindle-shaped mesoderm- 
cells. The aortic sinus, which arises as a split in the mesoderm, is shown at 
a. Ss. in figure 11, which cuts it in the region of the foot and again close to 
the pericardium. The right kidney, which is functional in the adult, is shown 
in figure 11 at k. It, as well as the abortive left kidney, arises as an evagi- 
nation from the pericardium, while its duct, which is far to the right of the 
plane of the section, arises as an invagination of the inner surface of the 
mantle where this becomes continuous, on the extreme right, with the body- 
wall. 
THE DEVELOPMENT OF THE ORGANS OF RESPIRATION. 
In the adult these are on or under the inner surface of the mantle, and 
consist of the lung, the right gill, and the osphradium; and they stand in 
intimate anatomical relation to the heart and the renal organ. The lung is 
a large, elliptical, thin-walled pouch which opens into the chamber of the 
mantle through an aperture that is protected by valves. The opening is on 
the left side of the mantle above the left siphon and posterior to the osphra- 
dium—a small, oblong, laminated organ, about 3 mm. long. The gill ex- 
