Parasites and Parasitisms. 3 



the preservation of the species. Among these are the taeniae 

 which spend their larval or cystic stage in one animal and their 

 mature stage in another which prej's upon the first. The trema- 

 todes in their larval stage infest a'mollusc and as adults they live 

 in ruminants and other mammals. The echinorhynchus as a 

 larva infests the May beetle, and as the mature worm the pig 

 which eats the beetle. These are compulsory parasites, but not 

 permanent ones. 



Intermittent Parasites embrace such as come to the host for 

 nourishment and then leave it at once. The diptera furnish 

 many such examples : mosquitoes, gnats, simulidae, tabanidae, 

 haematobia, stomoxys, which either draw blood or live on the 

 secretions. 



The term remittent has been applied to parasites which breed 

 away from their hosts and come upon them in companies often at 

 given sea.sons. We have examples in lice, fleas, woodticks, and 

 leeches. 



Erratic Parasites are such as infest not one species or genus 

 but two or many, the choice being made in the individual case by 

 opportunity. Thus most predatory diptera, some acari and 

 ticks, fleas, bedbugs, and the parasitic fungi belong to this class. 



Monoxenous Parasites (monoy one, xenos hcstj are such as 

 live only in one genus or species of ho.st, and though the ovum 

 may pass out with the excrement, it or the embryo is taken in 

 again in food or water by another host of the same genus in 

 which it developed. 



Heteroxenous Parasites (eteros different, xenos host) are 

 such as pass different stages of their lives in different hosts 

 usually belonging to different genera. Several of these have been 

 named above under temporary para.sites. Among other examples 

 are : trichina that leaves the parent worm in the bowels, encysts 

 itself in the muscles, which must be eaten by another host in 

 order to its arrival at maturity in its intestines : also linguatula 

 which spends its larval stage in the lymph glands of the sheep, 

 and its mature existence in the nasal sinuses of the dog which 

 devours the sheep. 



PARASITISMS : THEIR RKLATIVE GRAVITY. 



In prognosticating the gravity of an attack, or an epizootic of 

 any one of the different species of parasites, one must take into 



