Bees, which give us their wax and their honey in exchange for the shelter which we afford them, have a mortal enemy, an acarus, which attaches itself to them, not in order to gain any advantage from them, but to cause their death. It is not so much a parasite as an assassin, and we may be excused from describing it. We have found acaridæ on certain polyps, the _Campanulariæ_ and _Sertulariæ_ of our coasts, and some years ago we described one which is very curious, and inhabits the southern whale, in the midst of its Cyami and Tubicinellæ. The anodonts of our ponds, as well as the _Uniones_ usually have the skin of their feet and that of their mantle encrusted with acari of every age, to which the name of _Atax ypsilophora_ has been given. The species which live on the anodonts are not the same as those which inhabit the _Uniones_; and Mons. E. Bessels, who has so fortunately returned from his voyage to the North Pole, on board the _Polaris_, 137 has seen the species of the anodonts crossed with those of the Uniones. [Illustration: Fig. 25.--Cheyletus eruditus.] There are also Arachnida which are parasitical only while young, as the _Trombidions_ and certain _Hydrachnæ_ (Fig. 24) which frequent aquatic animals. The _Leptus autumnalis_, known in France, at least in some localities, by the name of _Rouget_, is an acarian which throws itself upon man, and especially attaches itself to the roots of the hair: fortunately, it is only found in the country districts. The _Acarus_ (_Cheyletus_) _eruditus_ (Fig. 25) lives in books and collections, as well as on fruits and all kinds of bodies more or less damp, left in dark places; it has been studied by Van Der Hoeven. Mons. Leroy de Méricourt found in pus, which was running from the ear of a sailor, acaridæ which Mons. Robin refers to the genus _Cheyletus_, rather than to that of the _Acaropses_. [2] Hardy, in his _Leçons sur les maladies de la peau_ (Paris 1863), devotes a special chapter to parasitical diseases, and gives the complete history of the itch-mite. CHAPTER VII. 138 PARASITES FREE WHILE YOUNG.