TEEMATODA. 15 



of an inch in length. Tlie largest species vary from one to five 

 inches in longitudinal diameter.* 



Habits. — The Trematoda, or flukes, are not parasitic during the 

 entire period of their existence, for while passing through the cycle 

 of their life-development, they frequently change their residence, at 

 times inhabiting either open waters or the dewy moisture of low 



Fig. 1. — DiSTOMA minutum, Cobbold. — From the intestine of an oyster-catcher (Resmatopns 

 ostrealegus, L.) X 180 diam. — Original. 



pasture grounds. They perform active and passive migrations 

 from parasitic to non-parasitic abodes ; these phenomena becoming 

 strikingly suggestive when we take into consideration the compli- 

 cated series of facts which their various phases of development 

 unfold. During their larval wanderings in search of a final resting- 

 place which shall prove suitable to their adult condition, they pro- 

 visionally occupy the bodies of different kinds of evertebrata ; and, 

 therefore, in order to complete the genetic cycle of the trematode- 

 hfe, there must needs be a contemporaneity of both vertebrate and 

 evertebrate types — a concurrence which, surely, no reasonable 

 person can ascribe to mere fortuitous circumstances. These 

 remarks are likewise more or less applicable to all the other 

 helminths. 



Distribution. — In Mammals. — Flukes are sparingly found in 



* In making this statement I exclude Van Beneden's singular species Nematohothritim 

 filarina, wiiicli, if accepted as a fluke, would give us a Trematode three feet and three 

 inches in length. I have placed this genus in the Monostomidas. For a description of 

 the worm, see his Memoire sur les J'ers Intestinaux, y>- 108. 



