CHAPTER IX 

 MOLLUSCA 



Classification adopted 



Solenogastres 



f Isopleura Polyplacophora 



Gastropoda I Prosobranchiata ' Aspidobranchiata 



I J. J. VkJW WJ. CV A A V-AJ.ACH UCV i -_- * . - 1 j 



. , L~ . ., , , . iPectmibrancniata 

 [Anisopleura Opisthobranchiata 



[Pulmonata 

 Scaphopoda 



Pelecypoda (Lamellibranchiata) 

 Cephalopoda 



ONE of the most surprising facts brought to light by the study of 

 embryology is the near affinity of the Mollusca and Annelida. The 

 adult structure in the two groups is widely different, but in their 

 early development Mollusca and Annelida correspond almost cell for 

 cell. The credit of having proved this important fact, by the most 

 elaborate and laborious researches, belongs to the American School of 

 Zoologists. Conklin's ( 1897) monograph on the development ofCrepidula 

 may be regarded as the foundation of our knowledge on this subject. 

 A Polish zoologist, Wierzejski (1905), has, however, made the most 

 complete study of the cell-lineage of a Mollusc as yet published. He 

 worked on Physa, but Physa is unfortunately a form whose develop- 

 ment is very much modified, and hence is not suitable as a type for 

 special description. On the other hand, the common limpet, Patella, 

 exhibits a most primitive type of development, and it is found all 

 round the British and European coasts in countless numbers, and 

 allied genera are common on the coast of America. The development 

 of the Mediterranean species, Patella coerulea, has been investigated 

 by Patten (1885) and E. B. Wilson (1904), and we therefore select 

 it as a type for more special study. 



PATELLA 



There is no reason to believe that the development of the British 

 species, Patella vulgata, differs in any important respect from that 



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