ANIMAL PARASITES. 



"Whenever an animal/' says Leuckart, 1 "is too small and 

 too imperfectly armed to overcome and destroy another living 

 being upon which its instincts direct it to seek for nourishment, 

 it must be contented with robbing it, by feasting upon its blood, 

 juices, and solid parts." The only animals which occur as para- 

 sites in or upon the human body belong to the classes of Insects 

 and Worms, and perhaps also to the Infusoria. As far as 

 we yet know, these parasites of man are not subject to the 

 attacks of secondary parasites. Many of them are common 

 to Man and other Mammalia, whilst others are peculiar to him. 

 From the presence or absence of transverse strise in the muscles 

 they may be divided into two large groups. 



FIRST GROUP. 



PARASITES WHOSE MUSCLES EXHIBIT NO TRANSVERSE STRIATION. 



This group, with the exception of the still questionable Infu- 

 soria, is composed of the true Helmintha of authors. W r e will 

 first take into consideration the peculiarities in the mode of life 

 of these animals in general, and afterwards in detail. 



The Infusoria are destitute of all high organization. They 

 are simple, vitalised, membranous structures, which live by mere 

 endosmose. 



The other parasites of this division also, or the true Helmintha, 

 are destitute of many of the organs possessed by the higher 

 animals. Thus, they have no separate respiratory organ, and 

 the oxygen necessary to their existence, as to that of all organized 

 beings, must always be taken up in a dissolved state, simultaneously 

 with their fluid food. Hence they are enabled to exist within 

 the human body, either in its closed or open cavities, or in its 

 tissues. They constitute the true Entozoa, and furnish no repre- 

 sentatives to the series of human Epizoa, although some of them 



1 Vierordt's ' Archiv,' 1852, article " Parasiten und Parasitismus." 



