Ward. — Fish Parasites and Parasitic Diseases 209 



and spread of disease. (For a fuller discussion consult 

 Ward, 1905; see list at the close of this paper.) 



Animals may be merely mechanical carriers of disease 

 transporting the causal organisms from one point to another, 

 as flies transport typhoid germs or eggs of parasitic worms. 

 They may be breeders of disease when the germs go through 

 processes of multiplication within the body of the carrier 

 and the number of infecting organisms is greatly increased 

 thereby. This is the case with the germs of malaria, which 

 multiply in the body of the mosquito. In this instance the 

 latter is an essential condition for the spread of the disease; 

 it is hence a breeder as well as a transmitter of disease. 

 Finally animal organisms may be definite producers of 

 disease; and many among the various types of animal para- 

 sites belong in this class. 



To comprehend rightly the standing of parasites as 

 disease-producers, it is necessary to review briefly the ef- 

 fects which the parasite exerts on its host. Here again 

 the limits of the occasion demand extreme conciseness and 

 I may refer those especially interested to a more extended 

 discussion of the topic which I have published elsewhere 

 (Ward. 1907). 



Among the vertebrates, the only instance of a species 

 which definitely causes disease is that of the lamprey. Its 

 well known action on the outside of the body is distinctly 

 analogous to that of the hookworm in the intestines. 

 Usually disease-producing animals are small or at least gain 

 access to the body of the host in a stage of development 

 which is insignificant in size. Among the Crustacea and 

 Insecta one finds some groups highly modified to adapt them 

 to external parasitism and certain of these will demand at- 

 tention later in this paper; there are, however, only a few 

 of them that are internal parasites. The worms furnish 

 some ectoparasites on water-living hosts and a large number 

 of endoparasites which infest all types of animals and pro- 

 duce many serious diseases. But in the group of minute 

 single-celled animals, the Protozoa, are found the most 



