There are different leeches which inhabit invertebrate animals. Rang mentions a little creature of this kind in Senegal, living as a parasite upon the respiratory apparatus of an anodont. Gay discovered in Chili one of the Hirudinidæ in the pulmonary sac of an Auricula, and another on the branchiæ of a crab (_Branchiobdella Chilensis_). Mons. Blanchard has noticed a malacobdella in the branchiæ of the _Venus exoleta_; and it was known in the last century that the _Mya truncata_ of our coast also lodges a malacobdella which lies always under the foot of the animal. This is the hirudinean of which we have spoken above, which is allied transitionally to the trematoda. Together with the Hirudinidæ, we find very small worms, transparent, bristling with daggers and spikes of every form, which are found everywhere in fresh water. They are known by the name of _Naïs_. They are so completely transparent that we can see the action of all their organs through the substance of the skin. They have been the subject of several remarkable works. They live freely among the leaves of Lemna and other aquatic plants; but there is one species much more restricted in their habitat than the others; these seek assistance from the Lemneæ, and live at their expense. It is because of this kind, of which the genus _Choetogaster_ has been formed, that we mention them here. Their long bristles are veritable halberds, which they employ with astonishing skill, both in attack and defence. Among free parasites are found many very important articulated 115 animals, which neither the naturalist nor the physician ought to ignore. Some of these increase with frightful rapidity on the skin which harbours them, and their name alone is sufficient to inspire disgust, if not horror: others live like leeches at the expense of different animals, but without inhabiting them. There are many of these which follow their host everywhere, and which are dreaded not without just reason. Of this kind are gnats, fleas, lice, bugs, and a great many others, among which we ought not to forget the acaridæ, nor those singular parasites of bats, which bear no slight resemblance to spiders swimming in the midst of the fur. Volumes might be written concerning the organization and the habits of these parasites. These small creatures inspire the naturalist with no more disgust than the earth-worm of our flower-beds, or the salamanders of marshy places. Each one plays its part according to its conformation, and the most abject in appearance is not always the least useful.