^D. bilamellata.^ In /Doris/ we find a quite different type of nudibranch from that presented by the two genera /Dendronotus/ and /Æolis/. In one there were numerous branched cerata or dorsal papillæ, in the other many plain cerata, and in neither case were there any specialized [pg355] breathing-organs. In /Doris/, however, there are no dorsal papillæ at all, the back being covered with calcareous spicules, which form a sort of secondary shell. There is a circlet, or rosette, of retractile plume-like gills, or branchiæ, placed in the middle of the back posteriorly. There are various species of /Doris/, and of closely allied genera belonging to the family /Dorididæ/, which are to be found along the New England coast in tide-pools and among rocks. Those seen by the writer never appeared to be other than very sluggish creatures, and proved to be rather unsatisfactory tenants of the aquarium. [Illustration: /Doris bilamellata./] When discovered at low tide upon moist seaweed or stones, or when disturbed in the water, nudibranchs often appear like small lumps of jelly-like tissue, without a single attractive feature. It is only when they are placed in a jar of sea-water and left unmolested for a few moments that they unfold their beauties to the view. On the Pacific coast there are numerous species of nudibranchs, occupying corresponding shore stations and exhibiting the same degree of high coloration. There are over a thousand species of sea-slugs scattered about the various seas. They find their greatest development in tropical waters, and for brilliancy of tint and variation in form are unequaled by any other class of animal life, save perhaps the butterflies. ORDER =PROSOBRANCHIATA= To this exceedingly large and comprehensive order of gasteropods belong the great majority of univalve mollusks. In point of number of genera and species and in abundance of individuals, the prosobranchs probably exceed all the other orders of mollusks combined. They may be called the typical gasteropods, the shell being univalvular and generally spiral, and the animal exhibiting the singular torsion of the visceral mass which has placed the breathing-organs forward of the heart. They are provided with an operculum (placed upon the posterior dorsal portion of the foot), which is used to close the aperture of the shell against [pg356] enemies when the animal retires within its fortress. They are all marine except a very few families, which, supposedly of marine derivation, have become terrestrial in habit.