The branchiæ and feet are, of course, most largely developed upon the thorax.
Those upon the abdomen continue small, and those upon the tail or sexual body dwindle in size and change into other organs, such as fins, sacs, filaments, pincers.
3262. The number five of the branchiæ is also exhibited in the feet. There are mostly five large pairs of feet on the thorax, and as many small ones on the abdomen, as in the Crabs.
On the tail they appear more dwindled in size, and are frequently reduced to a smaller number.
Each part of the body has properly five rings; and these parts being the thorax, abdomen and tail, there are thus fifteen rings in all.
In the Crabs it is distinctly demonstrated that the maxillæ are nothing else but feet; there are therefore for the most part, after taking into account sundry arrests and coalescences, five pairs of maxillæ also.
The Crabs have usually two pairs of antennæ, whose signification is unknown; one pair is probably the elongation of the ear.
All Branchial animals have eyes, frequently too supported upon articulated pedicles.
They have a double nervous cord upon the ventral surface of the body.
They have, like the Mussels, a heart upon the dorsal aspect, and arteries with veins.