Trematoda 157 



method by means of which a parasitic flatworm may be maintained in 

 artificial media. Attempts to grow the parasites in vitro have resulted 

 in failure, largely because there is no adequate knowledge of their meta- 

 bolic requirements. Their physiology has been studied very little and 

 the factors which determine host-parasite specificity are quite unknown. 

 The basis of the relationship is chemical and the adjustment has de- 

 veloped gradually during a long period of association. Accordingly, the 

 only course of procedure is to maintain these worms in or on appropriate 

 hosts. The life cycles of most species consist of two or more successive 

 generations which may infest different host species. Certain parasites 

 manifest very close host-parasite specificity while other may complete 

 their development in a variety of different hosts. All members of a 

 natural family follow a similar course of development and it has be- 

 come clearly evident that types of life cycle are closely correlated with 

 phylogenetic and systematic relations of the worms. A brief account 

 is here given of attempts to culture the trematode, Cryptocotyle lingua, 

 and the cestode, Crepidobothrium lonnbergi. C. lingua was selected be- 

 cause this species manifests little host-parasite specificity, and C. lonn- 

 bergi because it is relatively common and, being a parasite of cold blooded 

 hosts, may be studied at room temperature. 



The writer (1930, 1932) has reported attempts to culture Cryptoco- 

 tyle lingua and Crepidobothrium lonnbergi. The metacercariae of 

 C. lingua were washed in dilute seawater and placed in an isotonic salt- 

 dextrose medium. As a result of a series of experiments it was determined 

 that a pH of 6.8 is the optimum hydrogen-ion concentration for the sur- 

 vival of the worms. Specimens remained alive in this medium for 12 

 days, the medium being changed daily. During this time the young 

 worms did not develop; on the contrary they slowly diminished in size 

 and at the end of the experiment were only about % as large as when 

 removed from their cysts. It seems that the tissue of the body was utilized 

 in metabolism and that the specimens actually starved to death. When 

 kept at 38 , the worms lived only 6 days, but this result is significant, 

 since sexual maturity is attained in the vertebrate host in about 6 days. 

 Keeping the worms under reduced oxygen pressure did not appreciably 

 alter the degree of activity or time of survival. The worms may live for 

 long periods of time in solutions from which the oxygen has been re- 

 moved. 



To supply accessory food substances, veal was digested and the result- 

 ing extract was filtered, adjusted to a pH of 7, and sterilized. Various 

 amounts of this material were added to the salt-dextrose solution to form 

 a culture medium. With the addition of protein material, the media 

 rapidly disintegrates as a result of bacterial growth. The worms did not 

 develop at room temperature. In one experiment the worms were put in 



