Facultative, or pseudoparasites are species which are normally- 

 free-living, but which are able to exist as parasites when accidentally 

 introduced into the body of another animal. Ex., vinegar-eels, 

 larvae of blow-flies. 



Spurious parasites are objects which have been mistaken as para- 

 sites and, sometimes, even been given specific names. Ex., plant 

 hairs, bits of gristle, banana cells, etc. Grosser illustrations are 

 cases of frogs and lizards claimed to have lived within the human 

 body. 



Structure of the parasite — Adaptation to environment nowhere 

 more strikingly shown than in the case of parasites. With all the 

 variety of structure, there are yet certain characteristic modifications 

 which accompany parasitic life. 



Adaptations of two general types: 



A. Neoformations — Organs of attachment, as, modified tarsi of 

 hair-inhabiting lice; suckers of leeches, tapeworms, et al; hooks of 

 various intestinal parasites. Adaptations in sexual organs, illus- 

 trated by hermaphroditic forms. Adaptations in eggs of parasites. 



B. Degenerations — Reduction of organs of locomotion, as, loss of 

 wings in parasitic insects; of legs and other appendages of Crustacea; 

 fusion of body-segments; loss of cilia. 



Correlated changes in the muscular system. Reduction of the 

 nervous system and sensory organs. 



Loss of alimentary canal. 



While these various changes are usually spoken of as degenerations, 

 it must be remembered that they all have for an end the better 

 adaptation of the parasite to its manner of life. 



Occurrence of parasites — Practically all animals subject to attack — 

 From amoeba with parasites in its nucleus, or in its body -protoplasm, 

 to man. Leuckart, '76, lists 33 species parasitic in man but now 

 (Neveu-Lamaire, '08) over 150 species infecting man are known. 



Parasites themselves may be infested by other parasitic species 

 which are then known as hyperparasites. 



Numbers which may occur in a single host — In the case of protozoan 

 parasites of the higher animals there may be countless myriads of 

 individuals. Of higher forms of parasites a crow examined here had 

 as many as 2,000 nematode worms in a single drop of blood; chicken 

 had over 1,000 tape worms. 3500 hookworms have been expelled 



