PAKAS1TES AXD THEIR DEVELOPMENT. 201 



extracting the parasite by gentle traction, and of avoiding 

 the infliction of any injury to its body. This latter forms, 

 in fact, the great desideratum of the operator since, if the 

 body of the parasite be broken, and a portion left still 

 within the body of its host, additional and it may be serious 

 irritation is thereby set up. The long and slender body of 

 the worm is accordingly wound slowly and carefully around 

 some object, and the negroes of the Guinea coast are said 

 to be dexterous and skilful in the performance of this some- 

 what delicate operation. 



Perhaps one of the most remarkable points in the 

 history of parasites is that which refers to the geographical 

 distribution of certain of their numbers. That parasites 

 require to be provided with certain appropriate conditions 

 for development is a fact already noted. Indeed, we may 

 go much further and say that the conditions demanded for 

 the successful development of many of these animals are 

 infinitely complicated, and are in many cases of singularly 

 curious nature. But it would also seem that in their " dis- 

 tribution" over the surface of the globe, and in their 

 selection of certain countries or regions as especial spheres 

 of development, some parasites evince remarkable traits of 

 character. One of the best known instances of this fact 

 is afforded by a species of tapeworm, to which the somewhat 

 uncouth to ears unscientific, at least name of Bothrio- 

 cephalus has been given. This latter is a species of " broad- 

 headed" tapeworm, differing from its common neighbours 

 in special points. It is unquestionably the largest or longest 

 parasite which invades the human territory, and may attain 

 a length of over twenty-five feet ; its average breadth being 

 about an inch or rather less. In a large "broad-head," as 

 we may call it, upwards of four thousand joints or segments 

 may exist, and as each joint after the first six hundred 

 is capable of producing eggs and embryos, this foreign 

 neighbour is seen to be fully as productive as its commoner 

 relations. The most interesting fact regarding the " broad- 

 head," however, relates to its geography and to its exact 



