The creatures composing this order approach nearest to the Vertebrate animals of all the Mollusca. They have a distinct brain included in a box of cartilage—the vanishing remains of a bony skull; they have large highly-coloured and complex eyes, protected in some species by eyelids; and ears of simple structure, hollowed in the cartilage of the rudimentary skull. They are remarkable for having three separate and well organized hearts, one for the circulation of the arterial blood through the body, the others for the projection of the venous blood through the two gills.
Any person who has had an opportunity of examining one of these animals in a living state, must have been struck with a very curious phenomenon. Over the whole surface of the body there are coloured spots which are perpetually changing their position and figure, running into each other and separating, playing hither and thither, contracting and dilating, appearing and disappearing, with great velocity and in the most singular manner. On close examination, it appears evident that these changes are owing to a fluid which moves irregularly within the substance of the skin. Even after death the spots continue to play for a considerable time, and that on small portions of the skin cut away from the rest.The cause of this curious appearance is not yet