Crabs

copulate at the front parts of one another, belly to
belly, throwing their overlapping opercula to meet one another:
first the smaller crab mounts the larger at the rear; after he has
mounted, the larger one turns on one side. Now, the female differs
in no respect from the male except in the circumstance that its
operculum is larger, more elevated, and more hairy, and into this
operculum it spawns its eggs and in the same neighbourhood is the
outlet of the residuum. In the copulative process of these animals
there is no protrusion of a member from one animal into the other.