24 ESSENTIALS OF ZOOLOGY 



this direction may consult Ward and Whipple's ' Fresh Water 

 Biology,' Cash's 'Bay Society Monograph of the British Bhizo- 

 poda,' Saville Kent's 'Manual of the Infusoria,' and special 

 works like ' Nordisches Plankton.' 



Symbiosis. Protozoa and minute algae are sometimes 

 associated, with mutual benefit. In such an association, the 

 Protozoon is relieved of its waste material and supplied with 

 oxygen, and the alga is provided with all the food it requires, 

 including carbon dioxide, and it enjoys the protection furnished 

 by the cytoplasm of the Protozoon. Such an association 

 mutually beneficial is expressed by the word symbiosis. 



Parasitic Protozoa. All of the groups of Protozoa are 

 liable to a temporary, accidental appearance in the alimentary 

 canal of higher annuals. Many are destroyed by the digestive 

 juices, but some are able to survive and to pass through the 

 alimentary system without much change. A number have 

 found it possible to live and multiply in the alimentary canal 

 as facultative parasites, and others have become adapted to 

 living in the Metazoa and have changed into obligatory 

 parasites. 



Examples of these will be met with in the course of 

 dissection of the types which follow, and others are of a high 

 degree of importance from the diseases which they produce in 

 man and mammals. 



Amoebaea. An amoeba called Entamoeba histolytica has 

 been found in the intestine of man when affected by tropical 

 dysentery. It has an amoeba-like appearance, possesses a 

 small nucleus, and reproduces by fission and the formation of 

 chromidial buds. Several kinds have been described. 



Ciliata. Only one or two ciliates have been found in 

 association with disease; Balantidium coli has been obtained 

 from the intestine of man and of the pig. OPALINA is common 

 with accidental visitors in the large intestine of the frog. 

 Opalina is ciliated, has a thin ectoplasm with longitudinal 

 myonemes and several nuclei ; there are no food nor contractile 

 vacuoles, no mouth opening.. These negative characters are 

 associated with the food of the parasite being imbibed from 

 the food of the host. Opalina reproduces by transverse fission 



