PROGRESSION ANJJ RETROGRESSION. 



187 



water snaiJ. We need not follow liim into details. It will be enougli 

 for our purpose to note that from a " mulberry mass " the egg after 

 segmentation of the yelk there comes a sort of hemispherical cup. 

 The mouth of the cup changes from a circle to a long slit, and the 

 edges of the slit unite except at one point. The embryo has now 

 taken on the moUuscan type. The aperture along the line of the slit 

 is the opening to the sac, the mouth to the coming snail. The line 

 along Avhich the approximated sides of the cup have united is in the 

 trend of a plane which divides the body into right and left sides, equal 



Fig. 4. Symmetry. Embryotic Snail : m, mouth ; 7na, mantle ; c, creeping disk ; ?, intestine ; 

 7i, heart (.auricle and ventricle in line with the intestinal tube) ; r, remnants of yolk-cell. 



and similar. The mantle has begun to form, and as a sort of cap it 

 covers the part of the body opposite the mouth. The intestine begins 

 in a little depression under the mantle and in line with the mouth and 

 stomach. This depression is elongated, becomes a tube, and opens 

 into the stomach. A few days later, traces of a heart appear as two 

 pulsating, globular sacs, placed end to end (Fig. 5). 



If development were arrested at this stage, our snail would be bi- 

 symmetrical, and, if it had a shell, the shell would be in two equal 

 valves, right and left. But development goes on, and now every step 

 is a departure from right and left symmetry. First, the intestine gets 

 a, twist. Other organs are quick to follow. Even the heart moves 

 askance. The two chambers which, a while before, were placed end 

 to end in line with the axis of the body, begin to change position. 

 The receiving chamber moves obliquely to the right and downward, 

 the distributing chamber to the left and upward. The right fold of 

 the mantle spreads rapidly ; the left, not at all. The right side of the 

 body grows rapidly ; the left remains almost stationary. The right 

 valve of the shell grows rapidly, and twists over with the inclosed 

 body ; the left is completely aborted. Now, it is a very significant 

 fact that the only parts which do not share this one-sided overgrowth 

 are the head and creeping disk ; and these are the parts Avhich, not 

 being covered by the mantle, do not become incased in the shell. Ex- 

 posed to the water or the air equally on botli sides, they retain their 

 bilateral symmetry. 



