Page:Popular Science Monthly Volume 8.djvu/197

Rh 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 they reach the maximum of development, and appear as long, flexible limbs. In this order—represented by the Octopus—the molluscan type reaches the highest expression. Early in the history of life, the type had unfolded and found expression in Cephalopods of great bulk and of many species. The Cephalopods have long been a waning dynasty (Fig. 3).



As we have met the palpi—rudimental in the oyster—in other guise in oyster's distant relatives, so we will find that toughened portion, so faintly pronounced in the oyster, expressed with greater distinctness 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, we might look on the oyster as a degraded