There are many species belonging to several genera which frequent the Maine and Massachusetts coasts, but those most likely to be encountered are the following: GENUS ^Dendronotus^ ^D. arborescens.^ This species is about one inch long and variable in color, but is usually reddish-brown or rose. There are no tentacles, but in their place are two antler-like appendages pointing forward and branched like a tree. All along the back are two rows of these curiously branched processes, which give to the animal the appearance of a plant. These /cerata/, or dorsal papillæ, are delicately transparent, contractile, and richly colored. The function of these papillæ is not fully known. As the animal has no specialized breathing-organs, it is reasonable to suppose that respiration is carried on through the outer skin and perhaps all over the surface of these branched papillæ. The liver, which [pg354] in most nudibranchs is extremely large and completely surrounds the stomach, in /Dendronotus/ also extends into these dorsal cerata, so that they may have some sort of digestive function. [Illustration: /Dendronotus arborescens./] GENUS ^Æolis^ ^Æ. papillosa.^ This is probably the commonest nudibranch upon the North Atlantic coast, and it occurs as frequently in European waters. It may readily be found clinging to stones, algæ, the piling of wharves, eel-grass, etc., in bays or in rocky tide-pools. The color is yellowish-gray to orange, with purplish or olive spots. The cerata are very numerous, and cover the dorsal portion of the animal save for an open space along the middle of the back. There are two sets of tentacles, the two dorsal and the two simple labial tentacles. The foot tapers behind to a sharp point, and is truncate in front. As in /Dendronotus/, the liver is diffused into the cerata, which in /Æolis/ are simple tubular processes without the branching character of those of the former. Both species lay their eggs in a gelatinous, bobbin-like cord, which is hung in festoons over rocks or upon zoöphytes, or at times they wind the cord in a coil upon the surfaces of stones. When the young first hatch out, they are provided with a glassy nautiloid shell, a fact which would indicate that at some remote period they were, like most mollusks, provided with a shell. /Æolis/ swims in an inverted position, and is at times exceedingly active. It is very variable in coloring and in the number of cerata. [Illustration: /Æolis papillosa./] GENUS ^Doris^