[Illustration:  Limpet, Patella, cut transversely:  a, foot; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]

The third and highest class of Mollusks has been called Cephalopoda, in reference again to a special feature of their structure.  They have long arms or feelers around the head, serving as organs of locomotion, by which they propel themselves through the water with a velocity that is quite extraordinary, when compared with the sluggishness of the other Mollusks.  In these animals the head is distinctly marked,—­being separated, by a contraction or depression behind it, from the rest of the body.  The feelers, so prominent on the anterior extremity of the Gasteropoda, are suppressed in Cephalopoda, and the eyes are consequently brought immediately on the side of the head, and are very large in proportion to the size of the animal.  A skin corresponding to the mantle envelops the body, and the gills are on either side of it;—­the stomach with its winding canal, the liver, and heart occupy the centre of the body, as in the two other classes.  This class includes all the Cuttle-Fishes, Squids, and Nautili, and has a vast number of fossil representatives.  Many of these animals are destitute of any shell; and where they have a shell, it is not coiled from right to left or from left to right as in the spiral of the Gasteropoda, but from behind forwards as in the Nautilus.  These shells are usually divided into a number of chambers,—­the animal, as it grows, building a wall behind it at regular intervals, and always occupying the external chamber, retaining, however, a connection with his past home by a siphon that runs through the whole succession of chambers.  The readers of the “Atlantic Monthly” cannot fail to remember the exquisite poem suggested to the Autocrat of the Breakfast-Table by this singular feature in the structure of the so-called Chambered Shells.

[Illustration:  Common Squid, Loligo, cut transversely:  a, foot or siphon; b, gills; c, mantle; d, shell; e, heart; f, main cavity, with intestines.]

Cuvier divided the Mollusks also into a larger number of classes than are now admitted.  He placed the Barnacles with them on account of their shells; and it is only since an investigation of the germs born from these animals has shown them to be Articulates that their true position is understood.  They give birth to little Shrimps that afterwards become attached to the rocks and assume the shelly covering that has misled naturalists about them.  Brachiopods formed another of his classes; but these differ from the other Bivalves only in having a net-work of blood-vessels in the place of the free gills, and this is merely a complication of structure, not a difference in the general mode of execution, for their position and relation to the rest of the organization are exactly the same in both.  Pteropods constituted another class in his division of the type of Mollusks; but these animals, again, form only an order in the class of Gasteropoda, as Brachiopods form an order in the class of Acephala.