PROGRESSIOX AND RETROGRESSION. 185 



whose folds are not united along their edges. Near the mouth, on the 

 ventral side, is a portion of the surface a little tougher than the rest. 

 This toughened surface on the oyster we shall find as significant as 

 we found the softened vascular patch on the surface of the Pteropod. 



The leaf-like bodies which surround the mouth appear as silent 

 members. In some form or other they are present in all mollusks, 

 and in the order of Cephalopods tliey reach the maximum of develop- 

 ment, and appear as long, flexible limbs. In this order represented 

 by the Octopus the moUuscan type reaches the highest expression. 

 Early in the history of life, the type had unfolded and found expres- 

 sion in Cephalopods of great bulk and of many species. The Cepha- 

 lopods have long been a waning dynasty (Fio;. 3). 



Fig. 3. Octopus fulvus. 



As we have met the palpi rudimental in the oyster in other 

 guise in oyster's distant relatives, so we will find that toughened por- 

 tion, so faintly pronounced in the oyster, expressed with greater dis- 

 tinctness in oysters' nearer relatives. In the mussel this toughened 

 surface supports a bundle of fibres, which protrudes from the shell 

 and adheres to a rock or wharf-post. In the cockle we find this same 

 portion prolonged into a finger-like organ, which performs the office 

 of locomotion. It is called a foot. In the teredo this "foot" has 

 reached the maximum of development, as the palpi in Octopus. But 

 for the rudimental palpi, Ave might look on the oyster as a degraded 



