THE DEVELOPMENT OF PALUDINz\ VIVIPARA. 117 



carJiuiu, kidueys^ and ducts remain as iu tlie last stage^ but 

 tlie gonad is now clearly formed as a tliin cord of cells lying 

 beside tlie liver. 



The visceral connectives are now completely formed, and 

 are united by a commissure, wliicli in this stage lies asym- 

 metrically, that is, wholly below the original right portion of 

 the mantle cavity. This is, however, a secondary condition, 

 and probably due to a tendeucy to place itself in relation 

 with the symmetry of the external form, for a stage inter- 

 mediate between this and the last shows that the commissure 

 is formed iu part from the floor of the mantle cavity just at 

 the entrance of the genital duct. 



Fig. IG is a transverse section across the visceral hump 

 in the region of the kidneys and heart. Liver and stomach 

 lie ventrally, and above them is seen the pericardium, with 

 the heart at the extreme left of the section. Dorsally to the 

 pericardium is seen the kidney {k.) with its openings into 

 pericardium {jjc) and duct (A*, d.), both cut through. The 

 kidney duct is beginning to pass ventrally to the rectum [rec.) 

 as described above. The genital duct is cut through at 

 I. m. c, and the wall of the left kidney at I. h. 



Stage H (figs. 17, 18, 19), — A definite coil is now being 

 formed on the right side, about one complete turn of the spiral 

 having been made, and the old crooked setting of the hump 

 on the foot is nearly lost. In this stage the organs attain very 

 nearly their adult condition in all essential points. The ali- 

 mentary canal has increased much in length and become more 

 coiled. The oesophagus bends down and towards the right to 

 open into the stomach, which now stretches as a great sac 

 below the liver, opening into it dorsally, and forming at the 

 right-hand extremity a blind sac, which shows a tendency to 

 follow the coiling of the liver (fig, 17). The rectum now 

 opens out of the stomach quite ventrally and posteriori}^, 

 passes towards the left side of the body, then bends sharply 

 upwards behind the pericardium, and runs dorsally along the 

 roof of the mantle cavity, bending suddenly to the right just 

 before it reaches the anus. The liver is greatly developed 



