THE DEVELOPMKNT OF PALUDINA VIVIPAUA. 121 



It was then sectioned in a plane transverse to the long axis 

 of the visceral hump, and portions of these sections are por- 

 trayed in figs. 20, 21, and 22. The most noticeable feature is 

 the greatly developed visceral hump, which was held erect 

 over its head, bending in a decided manner at the apex, as 

 though forming the first turn of an exogastric coil. A 

 further remarkable feature is the perfect bilateral symmetry 

 of the whole embryo, though it is obviously at an advanced 

 stage of development, for though it is impossible to compare 

 it with any given stage in the normal course, the head is 

 well developed, the foot has found its normal creeping sole, 

 and a small operculum is already present. The general dis- 

 position of the organs can be made out from the drawings of 

 the whole animal. The stomach, it will be seen, forms the 

 apex of the visceral hump, while just below it an enormously 

 developed pericardium fills up for some distance the space 

 between the descending oesophagus and rectum. Paired 

 kidneys are seen at k and A:', but are better described in 

 connection with the transverse sections. The same is the 

 case for the great bulge in the lower posterior region of the 

 visceral hump behind the rectum, which might be taken for 

 the mantle cavity. Sections, however, show it to be merely 

 the continuation of a great space which surrounds all the 

 organs nearer the apex of the visceral hump, as seen in 

 fig. 20, which is a transverse section in the region of the 

 kidneys, and just below the pericardium. The mantle cavity 

 is shown in figs. 20 and 21 as a narrow and symmetrical 

 organ lying anterior (morphologically dorsal) to this great 

 space, and apparently compressed by it. Towards the apex 

 of the hump it forms two symmetrical horns Avhich run back 

 on either side towards the kidneys, but never fuse with them. 

 Here a wholly inexplicable condition obtains. The peri- 

 cardium, as already mentioned, is greatly developed, and it 

 is not surprising, in an embryo otherwise symmetrical, to 

 find out that here also two symmetrical evaginations have 

 been formed. These, which must be the kidneys, are, on the 

 one hand, very widely open to the pericardium, while on the 



