160 MOLLtTSKS. 



This depression deepens, its aperture narrows, and a more 

 or less spherical form is again assumed. It is obvious 

 now that the sphere has double walls, and that its cavity 

 has an open communication with the exterior. It is now 

 called a gastrula. 



Compare the gastrula, as to the type of its structure, 

 with a hydra stripped of its tentacles. 



The later stages within the egg are not such as are 

 common to most of the higher animals. They are difficult 

 of study, and they will not be described here in detail. 

 The student will readily see that the gastrula grows, 

 transforms into a creature less simple, and begins to turn 

 round and round in the albumen of its egg by means of 

 a circlet of cilia which it has acquired. As it increases 

 in size, dorsal and ventral surfaces become distinguish- 

 able, a transparent matrix of a shell appears, and a foot, 

 with a head imperfectly marked off at its anterior end. 

 A pair of blunt processes at the sides of the head fore- 

 shadow tentacles, and two very black pigment spots repre- 

 sent the eyes. A pulsating heart becomes visible through 

 the shell, and near it a tubular esophagus ; and by the 

 time it has eaten all the egg contents, and is ready to 

 come forth, most of the adult structures are recognizable. 



It will, no doubt, be more convenient to find all these 

 stages in eggs taken from different capsules than to try to 

 follow the transformations through with a single capsule. 

 If a goodly number of capsules are obtainable, it is quite 

 possible to find all these stages at one time. 



The young snail within the egg is called an embryo, 

 and all the transformations by which it comes, from being 

 a simple, undifferentiated cell, to the possession of perma- 

 nent and serviceable organs, constitute its embryology.* 



1 Try on' s Structural and Systematic Conchology is recommended for 

 reference, and for use in making a further study of mollusks. 



