There are a few Crustacea living habitually on the high seas which cannot be reckoned as belonging either to the true plankton or to the necton, since they depend on outside help for keeping themselves afloat. Among these are the Barnacles which cluster on logs of drift-wood, and are among the most important causes of the "fouling" of ships' hulls on long voyages. The stalked Barnacles of the genus _Lepas_ are especially common in such situations, and the characters of their larvæ have been already alluded to. Certain species of sessile Barnacles are constantly found attached to large marine animals. For example, _Chelonobia_ adheres to the shell of Turtles, while _Coronula_ and some allied genera are found on Whales. [Illustration: _PLATE XIX_ _Latreillia elegans_, ONE OF THE DROMIACEA WHICH RESEMBLES A SPIDER-CRAB. FROM THE MEDITERRANEAN. (NATURAL SIZE) THE GULF-WEED CRAB, _Planes minutus_. (SLIGHTLY ENLARGED)] The little "Gulf-weed Crab" (_Planes minutus_--Plate XIX.) is found clinging to floating drift-weed nearly everywhere throughout the temperate and tropical seas of the globe, and is especially common in the area known as the Sargasso Sea, in mid-Atlantic. It is occasionally drifted to the south coasts of the British Islands. In Sloane's "Natural History of Jamaica," published in 1707-1725, it is stated of the Gulf-weed Crab that "Columbus, finding this alive on the Sargasso floating in the sea, conceived himself not far from some land, on the first voyage he made on the discovery of the West Indies." A few other Crustacea also form part of the peculiar fauna which is associated with the Sargasso weed, notably a swimming Crab, _Neptunus sayi_, and two or three species of Prawns. All of these are coloured olive-green, like the weed among which they live. CHAPTER VIII CRUSTACEA OF FRESH WATERS The Crustacean fauna of fresh water is much less rich and varied than that of the sea. Although the number of individuals in a pond or lake may be enormous, they will be found to belong to a comparatively small number of species. All the subclasses of Crustacea with the exception of the Cirripedia have representatives in fresh water, but in most of them only a very few of the families and genera comprise truly fresh-water species. In spite of the comparative poverty of the fauna, however, it is of very great interest, more especially with regard to the problems of geographical distribution; and the ease with which specimens may be collected everywhere, and kept in small aquaria, renders it a particularly attractive subject of study for the amateur naturalist.