ioo Papers from the Marine Biological Laboratory at Tortugas. 



It is surrounded by a folded ridge, mf, the margin of the shell-gland. 

 In plate 3, figure n, 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 oesophagus, 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 (r, fig. u) arises from the posterior (ventral) border of 

 the oesophagus near the mouth, and extends under the ventral surface of 

 the primitive stomach. A pair of salivary glands that are not shown in the 

 figure arises from the oesophagus, 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 n, near the bottom of the figure; 

 although the opening through which the auricle, au., 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. j. in figure n, 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 n 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- 



