150 ANIMAL PARASITES AND MESSMATES. 



The lerneans are the most remarkable of all para- 

 sites with respect to 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 Penellse; 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 bal^nopter^.. on a Balsenoptera muscidus among the 

 Loffoden Isles ; the Lerneoniscus nodicornis on a dolphin ; 

 the great shark of the coasts of Ireland {Scimnus 

 glacialis) generally has a lerneah on its eye. My son 

 brought from Rio de Janeiro some Scomberidae, whoSe 

 skin is covered with penellsB ; 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 penellse. We have found 

 sometimes many individuals on the same fish, stretching 



