on shells living in the Black Sea, but we have species of Calypl ra?a 

 and Trochus on our own coast. 



Ccdyptrcm sinensis lays its eggs in pear-shaped capsules at- 



of the same brood develop at the same time. Development be- 

 gins with the total segmentation of the yolk. After it divides 

 into eight cells the blastoderm forms, which consists of a single 

 layer of cells, the result of the subdivision of the first four spheres 

 of segmentation, which grow over and envelop the yolk spheres, 

 thus forming the two germinal layers (ectoderm and entoderm). 

 The cells of the outer layer multiply and form the blastoderm, 

 from which the skin, mantle and external organs, as well as the 

 walls of the mouth arise, while, as in the articulates, the aliment- 

 ary canal with its dependencies, the liver, etc., arise from the pe- 

 riphery of the yolk cells, the central mass being absorbed. 



the ectoderm pushes in at a spot which becomes the ventral 

 side of the body. This is the primitive mouth. The anterior 

 part of this celiular heap is the first indication of the - head- 



the larva, and is also seen in the embryo fresh-water snails (I'nl- 

 monata). The sides of the primitive mouth form the two - sails " 



mollusks, and which was first noticed by Forskal, who wrote on 

 animals just a century ago. Finally, the posterior edge of the 

 infolding, which is also at first a little mass of cells, is the first 

 indication of the foot. The whole surface of the embryo is now 

 Fig 136 covered with cilia, and by their movement 



the embryo with its fellows, rotates in its 



; - ! - - 



/*•-. ■ r The next change consists in the growth 



■HBHHVJ and differentiation of the parts already 

 ^K/KSKf sketched out. The germ of the foot extends 

 ha.-kwani-, the mouth-opening deepens and 

 ™ becomes tube-like, the first indication of 



vehger of Caiyptraa. the pharynx (Vorderdarm). The next most 

 important change is the presence of a layer of cells between 

 the outer and inner germinal layers, which is called the middle 

 germ layer, with cells very unlike the outer layer, from which aW 

 developed the muscles of the foot and head as well as the heart 



