“You have not looked very carefully; why," he continued more earnestly, "you haven't even seen one of the most visible features of the animal, which is as plain before your eyes as the fish itself; look again, look again!" and he left me to my misery.

【翻译】“哎呀，你看得不仔细，” 他更加认真地继续说，“你甚至连这只动物最明显的一个特征都没看出来，它（该特征）在你眼前就像鱼本身一样清楚；再看一遍，再看一遍！” 他让我陷入痛苦之中。

plainly 是副词，不能充当is 的表语， 可能是作者失误所致，把形容词plain 写成了副词。

plain 的意思是：清楚的；清晰的。跟clear, obvious, apparent, evident, distinct 等是同义词。

原句改为：which is as plain before your eyes as the fish itself