What metaphors are used in Aspects of the Novel by E. M. Forster?

In his discussion of story in the novel, Forster utilizes a curious set of metaphors drawn from biology. He interchangeably describes the function of the story in a novel as either a "backbone" or a "tape worm." He uses the image of a backbone to explain the role of the story as the internal structure that supports all other elements of the novel. However, he suggests the alternative image of a tapeworm in order to express the idea that the beginning and ending of the story in a novel is arbitrary, just as a tapeworm has no specified length and no discernible head or tail.