IX 



MOLLUSC A 



305 



In Fissurella the development is essentially similar; in this 

 genus also there is a cap-like larval shell which is eventually cast off; 

 but in the expanded peristome a notch appears, due to a correspond- 

 ing indentation of the mantle 

 edge. As growth continues 

 this notch is pushed further 

 and further up towards the 

 apex of the adult shell, 

 because the indentation in 

 the mantle edge becomes con- 

 verted into a hole by the 

 formation of a bridge of tis- 

 sue across its lower end ; the 

 mantle edge in this way re- 

 acquires a. smooth, rounded 

 margin, and then secretes, in 

 later periods of growth, con- 

 centric unbroken rings of 

 peristome. 



In Haliotis the larval shell 

 persists throughout life and 

 the notch in the mantle edge 

 is permanent, and as a result 

 a row of holes in the adult 

 shell is produced, since partial 

 unions of its edges, which 

 secrete bridges of shell, are 

 formed across its upper and 

 older part. In Haliotis the 

 growth of the peristome is not 

 quite even on the two sides of 

 the mantle edge, and as a 

 consequence the embryonic 

 shell becomes pushed to one 

 side. This is the first indica- 

 tion of the spiral twist so 

 conspicuous in the shells of 

 most Gastropoda. 



Pig. 241. — Three views of the just-metamorphosed 

 Acniaea mrginea to show the formation of the 

 adult shell and the loss of the larval shell. 

 (After Boutau.) 



A, dorsal view of stage in which larval shell is 

 retained as an apical knob. B, lateral view of a similar 

 stage. C, lateral view of stage in which the larval shell 

 has been cast off. Letters as in previous figure. 



EXPEEIMENTAL EMBRYOLOGY OF PATELLA 



We now pause to consider a question which must have presented 

 itself to the mind of the reader when we described in detail the 

 typical seriation of cell divisions in Polygordius: namely, are these 

 cell divisions important as separating materials destined to form 

 various organs, or are they merely a means for effecting an 

 approximately even distribution of nuclear material ? 



No experiments have been made which will allow us to answer 

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