PARASITES FREE DURING THEIR ^YHOLE LIFE. 109 



from the higher kmd of leeches to those which Uve at 

 the expense of fishes, of crustaceans, and especially of 

 molluscs, we see that the desire of possessing a lodging 

 is developed hy 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 Hirudinidoe 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 Mala- 

 cohdellv, and they have been so ill-treated by Nature, 

 that it is necessary to submit them to minute investiga- 

 tion 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 hiru- 

 dinidse much lower in the scale than those which are 

 found in fishes, and especially in mammals. 



Vampires make use of the papillae 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.) 



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 



