358 Messrs. Koren and Danielsseu on the 



not, like Forskal, observe the shell ; but Sars was the first to 

 confirm Forskal's observations regarding the shell. 



At the point where the heart makes its appearance, we see 

 first of all a transparent, greyish, and finely granular mass, of a 

 nearly round form, and placed close to the common membrane, 

 which has assisted in forming the rotatory organs and the foot 

 above, and the mantle below. Some contractions soon show 

 themselves in the membrane, exactly at the spot where the greyish 

 exuded mass just mentioned occurs. AVe then perceive some 

 small and extremely fine muscular tubes, which indicate the di- 

 rection of the contractions. These become stronger and stronger, 

 and as no limitation has yet taken place, we see the contractions 

 extend over the mantle, the foot, and the rotatory organs. 

 During the contractions, the membrane at the spot where the 

 heart shows itself gradually acquires the form of a vesicle, the 

 outlines of which become more and more distinct, and in the 

 walls of which we discover several muscular tubes. By thus 

 becoming limited, the membrane forms the heart, which then 

 detaches itself from the rotatory organs, froui the foot and the 

 mantle, and is situated to the right on the back (fig. 2 d). The 

 heart takes a very oblique position, and is quite naked externally. 

 Subsequently its walls become stronger, and enlarge ; the mus- 

 cular tubes are multiplied, transverse tubes are formed, and it 

 becomes filled with a fiuid as limpid as water (fig. 3/). We have 

 often counted the pulsations, and found that they vary in rapidity; 

 we may usually count forty to fifty pulsations in a minute ; but 

 these strokes are not always regular, for after some feeble beats, 

 we may observe that the pulsation is stronger. It also fre- 

 quently happens that the heart suddenly ceases beating, and is, 

 as it were, in a state of repose for some time. After a rest of 

 this kind, the pulsation is much stronger. The primary tubes 

 of the heart are cylindrical and dilated in some places ; their 

 walls are excessively delicate and shining, and they refract light 

 quite differently from the rest of the mass. We have not ob- 

 served any fluid in these tubes, nor have we remarked any cel- 

 lular structure. In the rotatory organs we observed similar 

 muscular tubes, but here several are seen to approach each 

 other, and in many places it may also be remarked that they 

 ramify. This ramification becomes more and more abundant as 

 it approaches the periphery of the rotatory organs; and as 

 the finest branches cross each other, there appears a muscular 

 network which serves to move the organs, of which we have just 

 spoken, in all directions. Amongst these muscular ramifications 

 there are some small calcareous granules, scattered in the mass, 

 which strongly refract light. 



In our former memoir we stated that the eyes were formed at 



