This mollusk received the first title in allusion to the pretty fable which was formerly narrated of its sailing powers, and the latter title is given on account of the extreme thinness and fragility of the shell. It is remarkable that the shell of the argonaut is, during the life of its owner, elastic and yielding, almost as if it were made of thin horn. The two arms of the argonaut are greatly dilated at their extremities; and it was formerly asserted, and generally believed, that the creature was accustomed to employ these arms as sails, raising them high above the shell, and allowing itself to be driven over the surface by the breeze, while it directed its course by the remaining arms, which were suffered to hang over the edge of the shell into the water and acted like so many oars. In consequence of this belief the creature was named the argonaut, in allusion to the old classical fable of the ship Argo and her golden freight. The animal, or "poulp," as it is technically called, is a lovely creature despite its unattractive form. It is a mass of silver with a cloud of spots of the most beautiful rose color, and a fine dotting of the same, which heighten its beauty. A large membrane, which is the expanded velation of the arms, covers all. It has been definitely proved that the use of the expanded arms which cover the exterior of the shell is to build up its delicate texture, and to repair damages, the substance being secreted by these arms, and by their broad expansions moulded into shape. The larger figure in the engraving represents the argonaut while thus within its shell. While crawling the creature turns itself so as to rest on its head, withdraws its body as far as possible into its shell, and, using its arms like legs, creeps slowly but securely along the ground, sometimes affixing its disks to stones or projecting points of rocks for the purpose of hauling itself along. When, however, it wishes to attain greater speed, and to pass through the waters, it makes use of a totally different principle. Respiration is achieved by the passage of water over double gills or branchiae; the water, after it has completed its purpose, being ejected through a moderately long tube, technically called a siphon. The orifice of the siphon is directed toward the head of the animal, and it is by means of this simple apparatus that progression is effected. When the creature desires to dart rapidly through the water, it gathers its six arms into a straight line, so as to afford little resistance to the water, keeps its velated arms stretched tightly over the shell, and then, by violently ejecting the water from the siphon, drives itself by reaction in the opposite direction. The uppermost figure shows the argonaut in the act of swimming. * * * * * THE TRAP DOOR SPIDER OF JAMAICA.