Like the annelids, they do not change their form, and as they are only attached to their host for a short time, naturalists have not thought fit to place them among parasitical worms, or Helmintha. However, if we pass from the higher kind of leeches to those which live at the 109 expense of fishes, of crustaceans, and especially of molluscs, we see that the desire of possessing a lodging is developed by insensible degrees, and that the lower kinds, are by their form, their organization, and their mode of life, as dependant as the greater part of the helmintha. Thus we see Hirudinidæ on the Mya, an acephalous mollusc, incapable of quitting their place, firmly fixed on the walls of the stomach of their host, and living quietly at his expense. They are called _Malacobdellæ_, and they have been so ill-treated by Nature, that it is necessary to submit them to minute investigation in order to determine their parentage. The most well-known leeches are those which attack man and the other mammalia, but some are also found on other vertebrate animals, especially on fishes. Their organization is always proportioned to that of the host which they frequent; thus, the simpler their host, the lower is their organization. The mollusc harbours hirudinidæ much lower in the scale than those which are found in fishes, and especially in mammals. Vampires make use of the papillæ of the tongue, and also of their teeth, which act as so many lancets; leeches apply their toothed lip, saw asunder the epidermis, and with the mouth applied to a network of capillary vessels, suck till they fall off, intoxicated with blood. We give here the different appearances which the skin assumes after the bite of a leech. (Fig. 4.) [Illustration: Fig. 4.--Different forms of the bite of a Leech.] [Illustration: Fig. 5.--1. Sucker, open; _a._ jaws. 2. One of the jaws magnified.] [Illustration: Fig. 6.--Section of a Leech. _a._ anterior sucker; _b._ posterior sucker; _c._ anus; _d._ stomach; _æ._ æsophagus; _i._ intestine; _s._ glands of the skin.] Fig. 5 (1 and 2) represents the jaws; 1, the jaws in their usual position; 2, a single jaw, to show its outer edge, which is cut with teeth like a saw. Fig. 6 shows a leech with a section of its digestive tube. The 110 letters _d_ _d_ indicate the different cavities of the stomach, which are filled in succession. We see in the fore part, the anterior sucker with the mouth, and behind, the posterior sucker with the anus. At the 111 side of the stomach are seen traces of the glands of the skin.