Webby fingers propell the grindylow in its underwater environment; however, this feature severely hampers its movement on dry land, as the membranes are easily and painfully ruptured. This brings up the subject of the grindylow's essential...
Word, word, word...what's the word? Not fault, not downfall, too dramatic by a bit--weakness!
essential weakness, that is, the frailty of its body once it has lost the upper hand.
Neville's imagination is intrigued by webby fingers. How would he hold a quill if he had webby fingers? It would be difficult to make potions; he'd have trouble holding things. Snape wouldn't like him any more if Neville had webby fingers.
It would be awful nice to swim so well, though; Neville liked to imagine, sometimes, that he could be an athlete if someone would let him try and he worked on it hard enough. Neville likes to think about these things when he should be thinking about something else.
Nevillie draws a webby-fingered hand in the margin of his parchment and elaborates. He would have keen underwater senses, too; none of this slight muted-sound feeling he gets when he's nervous that makes it hard to listen to the professors when they give directions. He'd have eyesight like a shark, which he supposed had good eyesight, since they ate so much. He'd read about it.
Neville has fantasies of being strong and wild and brave and liked. He dreams of being like a wolf, prowling the school and being feared and admired. He remembers the kind gaze of brown eyes that bespoke an old, lone wolf's wisdom. He wants with all his heart to look like that one day, and he tries.
Neville looks at his hands, with little webs ending a slight centimeter above his hand, barely any webs at all. Webby fingers would be fun, but they'd be an awful lot to take care of.