RECENT WORKS OK THE EKTOZOA. 347 



sites are thus distributed because they affect peculiar races of man- 

 kind. But the guinea-worm has been met with " in individuals of 

 every race ; the only condition apparently necessary being that they 

 should have visited some of the parts comprised in its endemic range 

 within a certain period." So, in like manner, English and American 

 visitors to the continent occasionally return thence the involuntary 

 hosts of Bothriocephalus latus, which they have brought with them 

 as a pleasing memento of their stay in countries beyond the sea. 



The backward state of Helminthology in former times affords no 

 data for solving the interesting problem, whether during the progress 

 of years an increase has taken place in the catalogue of human JSn- 

 tozoa. Again, are these parasites capable of being rendered endemic 

 in animal species to which they were before strangers ? This is pre- 

 eminently a question of fact, which it is yet possible experiment may 

 enable us to answer in the affirmative. 



These enquiries necessarily lead on to others, touching the pre- 

 cise character of the relations between Helminths and their host, and 

 the diverse conditions under which each form is capable of existing. 

 The limits of range in structure or habit which the species of 

 Helminths present, and their apparent modificability by external 

 influences demand henceforth a diligent investigation. For here the 

 adjustment between the organism and its environment is of so 

 peculiar a kind, as to suggest the consideration of a most curious 

 series of cases to ail entering upon the study of the boundless subject 

 of variation. 



The great majority of human Helminths appear to be introduced 

 from without, and, when found in considerable numbers, their pre- 

 sence is usually traceable to obvious neglect of cleanliness, the 

 drinking of impure water, or the eating of imperfectly cooked 

 flesh, particularly beef and pork. The guinea- worm is, in all pro- 

 bability, transferred to our bodies by bathing the feet in the 

 waters which this parasite frequents, while in an early phase of its 

 existence. The neighbourhood of dogs, also, unless proper precau- 

 tions be taken, is favourable to the development of Tcenia echinococcus. 

 But, of all animals, feral or domestic, the common pig is, beyond 

 doubt, the most fertile nurse of human Entozoa. At least, two im- 

 portant parasites, Trichina spiralis^ and the tape-worm, would, there 

 is good reason to suppose, cease to infest us did nob this favourite 

 quadruped act the part of a communicating medium. Considering 

 the important aids to helminthogenesis thus afforded, we may be 



