This brightly colored picture of the gooseneck barnacle (Lepas anatifera) is from Edward Donovan's The Natural History of British Shells published in 1799.
Back then, and until the 1830s, barnacles were grouped together with mollusks, simply because they had shells as do most frequently encountered mollusks. Barnacles are actually arthropods, because they have jointed appendages like insects and their relatives. This post was about a barnacle on a snail shell.
Here is the title page of Donovan's book. It is available on Google Books.