^A. Wilcoxii.^ This species appears at times in vast numbers in the waters of Florida, until the sea may truly be said to be fairly alive with them. They swim lazily with a waving motion of the parapodia. They disappear as mysteriously as they come, and for months not a specimen will be seen. There is a variety of this Floridian /Aplysia/ which occurs at Cape May, but no sea-hares are to be found north of that point. The tropical Pacific furnishes an astonishing wealth of these creatures, belonging to many genera and species, and among them are some of the most beautiful of the invertebrate animals. SUBORDER ~NUDIBRANCHIATA~ The opisthobranchs are divided into two suborders, the second of which is called /Nudibranchiata/. The name is well chosen and very suggestive, for it means "naked or exposed gills"; but this anatomical feature is only one of the peculiarities of this suborder. The nudibranchs are commonly known as "sea-slugs"; for, like the land-slugs, which are also true mollusks, they possess no shell at all. That they at one time carried a shell is evident from the fact that they are born with a rudimentary testaceous covering, which soon afterward disappears. A striking peculiarity of the nudibranchs is that the conventional molluscan mantle is not usually apparent. Instead of seeing the usual flaps or folds of the mantle which more or less encircle mollusks, and which one seems to have a right to expect, [pg352] they are entirely absent, and the body of the sea-slug assumes in consequence a worm-like appearance. Other notable features of the nudibranchs are the great number of tentacular processes that usually project from the dorsal region of the animal, and, in many of the genera, an entire absence of gills. When the gills are present, as is the case in several of the nudibranch families, they are not placed along the side of the animal, where one would naturally look for them, but are arranged in the form of a rosette of plume-like processes situated in the posterior dorsal region, or, in other words, on the animal's back. Nudibranchs are commonly to be found all along the Atlantic and Pacific coasts of North America, and more especially in the colder waters north of Cape Cod. They are essentially littoral, and live upon algæ in shallow water, upon eel-grass in sheltered places, and in tide-pools, where there is more or less vegetation. They crawl about the fronds of algæ, or swim, foot upward, with a gentle and undulating motion, or, when caught between tides, may be seen clinging to the under surface of rocks. Protective resemblances have been so remarkably developed in the nudibranchs that they are not always easy to discover; indeed, one may actually be looking at one for some time without suspecting it to be other than some torn fragment of seaweed. * * * * *