TREMATODS I CESTOIDS. 469 



with a flattened disk, both of which are well adapted to 

 adhere to other bodies, and are the principal organs of 

 locomotion. Leeches abound in stagnant waters. 



CESTOIDS, OR TAPE-WORM FAMILY. This Family 

 embraces tape-like worms, narrow towards the head 

 and widening behind, which in their mature state live 

 only in the intestines of vertebrated animals. They 

 occur in all the classes of vertebrates ; and generally dif- 

 ferent species are inhabited by different species of ces- 

 toids ; and sometimes two or three species of cestoids 

 inhabit the same species of vertebrate at the same 

 time, and in some cases the same intestine. Some are 

 scarcely visible ; others, the largest, attain in some cases 

 the length of one hundred feet ! The width is nearly 

 an inch in some of the widest. The eggs of a cestoid 

 never hatch in the same intestine in which the cestoid 

 lives, but only after they have been taken into the stom- 

 ach of another and suitable animal. Thence the embryos 

 pierce their way into the blood-vessels, and are carried 

 by the circulation of the blood into various parts of the 

 body, where they develop into larvae called hydatids. 

 The so-called "measly pork" is pork containing these 

 hydatids, that is, measly hogs are such as have their 

 muscles more or less filled with the larvae of cestoids or 

 tape-worms ; and if the flesh of such hogs be eaten before 

 cooking, which kills the hydatids, the man or animal 

 eating it takes these hydatids into his intestines, where 

 they are sure to develop into tape-worms. And so in 

 regard to all animals which have tape-worms ; they get 

 them by eating other animals in whose tissues there ex- 

 ist hydatids ; and the way those animals afflicted with 

 the hydatids get the latter is, as stated above, by swal- 

 lowing with their food or drink some of the infinitesimal 

 small eggs of the tape-worm. Two hundred species of 

 cestoids have already been described, quite a number of 

 which inhabit man. 



