806 ZOOLOaY SECT, xii 



and auricles, and the fact that these organs are also paired in the 

 lower Gastropoda, seems to point to a common ancestor for Pele- 

 cypoda, Amphineura, and Gastropoda, which was bilaterally sym- 

 metrical, had a creeping foot, a simple shell, paired auricles, 

 kidneys, and gills, and no odontophore. 



While the leading feature in the evolution of the Pelecypoda 

 has been the splitting of the mantle into two halves and the 

 resulting bivalve shell, the most noticeable fact in that of Gastro- 

 poda, apart from the appearance of the odontophore, has been the 

 torsion of the visceral mass, producing a characteristic asymmetry. 

 In the Cephalopoda, on the other hand, the primitive bilateral 

 symmetry is retained, and the most characteristic special feature 

 of the group is the extraordinary modification of the foot into arms 

 or tentacles, and funnel. The class is raised far above the remain- 

 ing Mollusca by its wonderfully high organisation, especially of 

 the nervous system and the eye, and there is nothing to indicate 

 close relationship with any of the lower classes beyond the general 

 conformity to the molluscan plan of organisation and the presence 

 of an odontophore. The Cephalopods form, in fact, a singularly 

 isolated group. Pakeontology has not hitherto given any indica- 

 tion of their origin, and embryology is equally silent ; the absence 

 of a free larva, and the profound modification in development 

 produced by the enormous mass of food-yolk, sharply separating 

 them from all other members of the phylum. 



