Among these crustaceans, we must mention the _Caligi_ and the _Arguli_, which never become bloated, the _Ancei_, and probably other genera. Instead of the ungainly and unusual forms of true parasites, they all preserve, together with their fishing tackle and locomotive apparatus, their neat and elegant appearance. The sexes even differ only in size. They remain during the whole of their life what they are at the beginning; that is to say, charming in form, with a delicately-shaped corselet, numerous and slender claws, and are as graceful in their movements as when in a state of rest. The greater number of osseous fishes lodge Caligi on the surface of their skin. These fix themselves by means of strong cables, but without sacrificing their liberty. They are usually called fish lice. Fishermen, when returning from the northern fishery, generally find their vivarium full of these graceful vermin. It may be said that 73 the caligi are common everywhere, and that each species has its own caligi. The fishes of the family Plagiostoma, notwithstanding the hardness of their skin, afford food to some of these; they multiply so rapidly sometimes, that they cover their host as though they took the place of scales. The cod gives lodging to a charming species of a very beautiful shape, which in its turn, affords a resting-place to the Udonella. It is always attached to the ovisacs, and doubtless plays the same part as the Histriobdellæ, so that we shall find the Caligi attending to the toilet of the cod, and the Udonellæ in their turn waiting on the Caligi. [Illustration: Of the natural size.] [Illustration: _Caligulus elegans_ (fem.)] The name Arguli has been given to some crustaceans which resemble the caligi in size and in manner of life, and which principally frequent fresh-water fishes. The _Argulus foliaceus_ is the name of the species which has been known for the longest time, and which is most extensively found. It is to be seen on our pikes, carps, sticklebacks, 74 and on the greater part of our river fish. Mr. Thorell, in his monograph, mentions twelve species of Arguli proper, and four species of which he composed the genus _Gyropeltis_. Four are found in Europe, two of which are on salt-water, and two on fresh-water fish. Quite recently, Professor Leydig has made known another species living on the _Phoxinus levis_. Arguli are met with on the fishes of Africa, the Indies, and North and South America. Like the caligi, these animals spontaneously abandon one host, to go and attend to the toilet of another.