The lerneans are the most remarkable of all parasites with respect to 150 their physical degradation. They are met with on all aquatic animals, commencing with the cetacea, and extending to the echinodermata and polyps; but it is especially on fishes that they are most abundant. They live on the skin or the gills, and sometimes establish themselves in the nostrils and on the eye-ball. They often hang on the outside, but we find some which hide themselves in the substance of the skin, and have no communication with the exterior except by a narrow orifice. Some elegant lerneans, which resemble a living pen, are called _Penellæ_; their head is divided into several branches, which plunge like roots into the tissues and even into the bones, so that the head and all the body remain suspended, as well as the ovisac tubes, to a long and but slightly flexible neck. They live on the body and the eye of certain fishes; some of great size are found in the Indian sea, but the most remarkable are those which have been observed on the skin of some of the cetacea. The _Penella crassicornis_ lives on a hyperoodon; the _Penella balænoptera_ on a _Balænoptera musculus_ among the Loffoden Isles; the _Lerneoniscus nodicornis_ on a dolphin; the great shark of the coasts of Ireland (_Scimnus glacialis_) generally has a lernean on its eye. My son brought from Rio de Janeiro some Scomberidæ, whose skin is covered with penellæ; and the charming fishes so abundant on the Belgian coasts, which are called _Sprot_ by the fishermen of the country, often have round their eyes strings which might be taken for marine plants, and which are in reality only penellæ. We have found sometimes many individuals on the same fish, stretching from the head 151 to the caudal region by means of their oviferous tubes, which in certain seasons acquire a pale green tint. The true Lerneans, such as the _Lernea branchialis_, a species that was the earliest known upon the different Gadidæ, and which we have observed on the _Callionyme lyra_, greatly resemble the Penellæ, but their body and their head are much twisted, and with the coils of tubes which contain the eggs, you might take them for a ball of thread. (Fig. 31.) [Illustration: Fig. 31.--Lernea branchialis, attached to the gills of Morrhua luscus.] The Sphyriones called _Leistera_ have also a most singular form, and a new species has been recently observed on a fish from the Straits of Magellan. The _Conchoderma gracile_ lives on the branchiæ of the _Maïa squinado_, the sea-spider of the Adriatic, and Mons. W. Salensky of Charkow, found a copepod crustacean, the _Sphæronella Leuckarti_, in the egg-pouch of an _Amphitoë_. The latter parasite has very peculiar characters of conformation and embryonic evolution.