782 ZOOLOGY SECT, xn 



and shell have been lost by degeneration in the Aplacophora or the 

 odontophore in Pelecypoda ; and it would appear, therefore, that 

 the two groups are to be derived independently from some primitive 

 form. 



The facts that the pelecypod shell, at its first appearance, is 

 univalve, and that the foot of the Protobranchia is of the creeping 

 type and their ctenidia plume-like, suggest the derivation of the 

 class from a form resembling a simple type of Gastropod with no 

 odontophore and with undisturbed bilateral symmetry. The 

 Amphineura are also bilaterally symmetrical, with paired ctenidia, 

 kidneys, and auricles ; and the fact that these organs are also paired 

 in the lower Gastropoda seems to point to a common ancestor for 

 Pelecypoda, Amphineura, and Gastropoda, which was bilaterally 

 symmetrical, 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. Palaeontology 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. 



