Mollusks. 535 



sometimes to the right, at others to the left, and occasionally in a cir- 

 cular manner, as if moving on a pivot. The 4 form of the young is ge- 

 nerally of an oval character, but frequently varies. The part on which 

 the cilia are situated very soon gets lobulated in an irregular manner; 

 and although there are scarcely two alike, yet there is generally one 

 large lobe on each side of the tubular orifice, with one or two smaller 

 ones besides. The tubular opening soon enlarges and alters in shape ; 

 the external and transparent envelope becomes more solid and calca- 

 reous, forming, in fact, the shell. At this period of development the 

 shells assume a more definite shape, and one more characteristic of 

 the adult form ; the surface is sometimes moulded into folds, and ir- 

 regularly marked with minute triangular prominences. Through the 

 enlarged opening a portion of the internal animal protruded, answer- 

 ing to the foot of the adult. This part very soon assumes the form as 

 well as the function of the foot. At this time it will be found, that 

 the anterior portion of the foot is clothed with vibratory cilia, as are 

 also two thin leaflets situated between the shell and the back of the 

 animal, which are in constant flapping action. After the foot is pro- 

 truded the shell gradually alters in form, the anterior part of the lip is 

 prolonged forward, and the creature soon learns to travel. About this 

 period the angle on the left labial rim is formed, near its attachments 

 to the spire. This, as the creature advances towards maturity, is ad- 

 vanced anteriorly, and prolonged till it forms the tubular snout of the 

 adult by approximating to the opposite rim. At this period the plug 

 at the superior portion of the cup becomes ruptured, and the young 

 escape into the surrounding water, and seek shelter in the crevices of 

 the rocks. Even in this young state they have the habit of the adult 

 of seeking such situations as will leave them exposed to the air dur- 

 ing some portion of the day ; if they are kept in confinement, they al- 

 ways seek the edge of the vessel above the water-mark. 



All the ova contained in one vase are not equally developed, nor 

 have they all the same form. In some the yelk is nearly as large as 

 the investing shell, while in others it is a mere speck ; and it seems 

 wonderful that so small a point should confer life and activity on so 

 large a shell. As they are thus unequally developed, some are ex- 

 cluded or effect their liberation some considerable time before others ; 

 it will be found on examination that they escape at irregular intervals 

 of from one to three weeks from each other. The periods, however, 

 vary according to the seasons. During the winter they take nearly 

 six months in arriving at maturity ; but during summer and the early 

 part of autumn, they escape from the cases as early as three weeks 



