382 E. RAT LANKTSSTEU. 



cepting a large superficial concavity at the point in front of 

 the velum,where it is destined definitely to make its appear- 

 ance. At this stage, represented in figs. 10, 11, and 12, the 

 foot has pushed itself forAvard on that aspect of the embryo 

 which may be called the pedal or ventral, and towards 

 which the blastopore faces. The blastopore has a very dis- 

 tinct margin of epidermic cells, and is at this period ciliated. 

 This up-standing collar of ectodermal cells may perhaps be 

 taken to represent the proper anal invagination or " procto- 

 deeum" (so-called to balance the " stomodseum" or oral inva- 

 gination) which in Paludina absolutely coincides with and is 

 one with the blastopore. Thus " the rectal peduncle" or 

 " pedicle of invagination," as I have termed the narrow part 

 of the endodermal invagination nearest to the blastopore, 

 remains permanently open to the exterior. In Limnseus it 

 is closed up at an early period by the growing together of 

 the lips of the elongated blastopore at its posterior or anal 

 end. 



In Pisidium also it is closed at a very early period by the 

 entire obliteration of the blastopore. In both these genera 

 the "pedicle of invagination" is subsequently eaten into by a 

 new invagination of the ectoderm of small extent in these 

 Molluscs, but apparently capable of large development in 

 other classes of animals (e. g., Arthropods) which may con- 

 veniently be termed the " proctodseum." The " proctodseal 

 tract" of the alimentary canal, derived as it is from a second 

 invagination of ectoderm, must be distinguished (where it is 

 developed to an appreciable extent) from those tracts derived 

 from the archenteron or endoderm of the gastrula ; just as 

 the stomodaeal tract derived from the oral invagination and 

 sometimes loosely spoken of as pharynx, must be distinguished 

 from the archenteron. When the blastopore remains open 

 throughout development the proctodaeum, though reduced 

 in size, can still be followed as an upgrowth around the blas- 

 topore, and similarly it appears from Bobretzky's observa- 

 tions that the strangely opposed case of the coincidence of 

 blastopore and mouth does not prevent the development of 

 the stomodaeum as a new growth which has either the form 

 of an upgrowth around the sunken blastopore as in Fusus 

 (Bobretzky's Plate xii,) or as a caecal ingrowth at the point 

 of closure of the blastopore as in Natica (Bobretzky's 

 Plate x). 



The form which the embryo has now attained is such that 

 under a slight pressure it readily takes up one of the two 

 positions drawn in figs. 10 and 11 — either presenting to you 

 the dorsal pallial surface or the ventral pedal surface. When 



