POLYGYRA ALBOLABRIS AND LIMAX MAXIMUS 291 



siderable space being left betvv^een the ectoderm and endoderm 

 (pi. 25, fig. 6 and succeeding figures). 



Tbe body of tbe animal as first observed consists of a slight 

 swelling of the upper side of the cell mass (pi. 25, fig. 7). 



In the following descriptions it must be remembered. that the 

 ventral surface of the animal is uppermost. 



The swelling above mentioned very soon shows a tendency to 

 divide into two parts (pi. 25, fig. 9, and pi. 24, fig. 17), the anterior 

 part of which is the foot proper, the posterior part the mantle, 

 shell sac, etc. 



Even at this early stage the embryonic shell can be observed, 

 consisting of a few dark colored crystalline plates, not yet united. 



At this stage the mouth appears at the position formerly occu- 

 pied by the blastophore; which has disappeared or been closed, 

 and consists of an invagination of cells, bounded by lateral lips 

 (pi. 24, fig. 20). 



The shell sac and mantle enlarge much the faster proportion- 

 ally, as shown in plate 25, figures 11, 12. 



In figure 12, and more distinctly in figure 13, the development 

 of the tentacles is shown, which at this stage have the appear- 

 ance of flat circular disks. 



The body is very small; from its extremity a circular semi- 

 transparent appendage, the podocyst, is developed, consisting of 

 two walls, which are connected by reticulated muscular cells. 

 The body now rapidly develops, as shown in figures 15 and 16, 

 plate 25. 



The shell has also increased in size, consisting of numerous crys- 

 talline plates, not yet united. 



The ectodermal sac and podocyst have also increased in size. 

 The surface markings are now apparent, and the beating of the 

 heart is plainly visible beneath the mantle. 



The smaller tentacles, the lateral lips of the mouth, the odon- 

 tophore and the beginning of the alimentary canal have appeared; 

 the pedal sinus is also apparent. 



At this stage (pi. 25, fig. 17-21), the ectodermal sac and podo- 

 cyst have reached their greatest development, and hereafter 

 gradually dimmish in size till completely absorbed. 



