The great majority of Crustacea are hatched from the egg in a form very different from that which they finally assume, and reach the adult state only after passing through a series of transformations quite as remarkable as those which a caterpillar undergoes in becoming a butterfly, or a tadpole in becoming a frog. Many of these young stages were known for a long time before their larval nature was suspected, and it is one of the curiosities of the history of zoology that, even after the actual changes from one form to another had been observed and described in several Crustacea, many eminent naturalists refused to believe in the possibility of their occurrence. This scepticism was largely due to the fact that the common fresh-water Crayfish, when hatched from the egg, has practically the same structure as the adult, and it was assumed that other Crustacea were developed in a similar fashion. Although certain cases of metamorphosis had been actually seen and described by naturalists in the eighteenth century, these observations were forgotten or misunderstood till they were confirmed by Mr. J. Vaughan Thompson, a naval surgeon stationed at Cork, the first part of whose "Zoological Researches" was published in 1828. Thompson's statements were much disputed at the time, but they have been confirmed by subsequent research, and it is now known that the majority of Crustacea undergo a more or less extensive metamorphosis after leaving the egg, although, as will be seen later, there are many important exceptions to this rule. If a fine muslin net be towed at the surface of the sea on a calm day, and the contents turned out into a jar of sea-water, it will usually be found to have captured, among other things, clouds of animated specks, which dance in the water or dart hither and thither with great rapidity. Many of these specks, when examined with the microscope, will be found to be Crustacea. Besides adult animals belonging to various groups, such as the Copepoda, which pass the whole of their life swimming near the surface of the sea, there will be numerous larval stages of species which in their adult form live on the sea-bottom. The identification of the species to which the various larvæ belong is a matter of considerable difficulty, and, although the general course of development is now well known for all the chief groups of Crustacea, there are very many even of the common British species in which the larval transformations have not yet been worked out in detail. [Illustration: FIG. 25--LARVAL STAGES OF THE COMMON SHORE CRAB (_Carcinus mænas_--SEE PLATE IX.). (Partly after Williamson.) A, Young zoëa, shortly after hatching; B, megalopa stage; C, young Crab. A × 20, B and C × 10]