35] LIFE HISTOR Y OF TREMA TODES—FA UST 35 



(Fig. 85), and C. trisolenata (Fig. 132). The material within the cyst cell is 

 usually milky, semi-opaque, either homogeneous or granular. In most cases 

 the contents consist mainly of the rhabditiform granules, which are indif- 

 ferent to dyes. These granules are not attacked by weak acids or alkalies, 

 but are digested by strong acids or bases. It seems probable that they are 

 of a derived protein nature. 



The formation of the cyst varies in the types studied. In the majority 

 of the species the cyst is a firm envelope with a free space around the embryo, 

 more or less filled with a watery fluid (Figs. 41, 84, 118). On the other hand, 

 some of the cysts are more mucoid in structure, with a granular viscous inner 

 portion and an outer jell (Fig. 11), 



The method of encystment differs in different species. In the monostome 

 where the encystment is rapid and an entire lot of mature cercariae encysts 

 in two or three minutes after they are freed from the redia, the process is so 

 rapid that the tail is not entirely cut off until the major portion of the cyst 

 as formed (Figs. 9-11). Where the time of encystment is considerably longer, 

 OS in C. trisolenata and C. dendritica, the tail is discarded long before the process 

 of encystment actually begins. Encystment is an adaptation on the part 

 of the larva to the change in environment. In two cases studied it takes place 

 within the liver of the primary host (C. micropharynx and C. biflexa). It is 

 highly probable that in these forms encystment was the result of temperature 

 stimuli, since the worms were secured in November when the winter had 

 already set in. It is apparently an adaptation for "wintering over." 



In only one group, the furcocercariae, has there been no record of encyst- 

 ment of the cercaria. Here the tail is dropped only under the pressure of the 

 cover slip or when the worm begins to disintegrate. La Valette (1855:34) 

 expresses this condition for his forms when he writes of the larvae: "Cercariae 

 cystibus non indutae in animalium vertebratorum intestinis pereunt." 



The phenomenon of encystment is one which has been observed by many 

 investigators. It has been seen and described very accurately by La Valette, 

 Moulinie (1856), Pagenstecher (1857), and a long line of later investigators. 

 Moulinie refers to the studies of earlier workers on encystment, mentioning 

 von Siebold (1835) and Steenstrup (1842). But the credit for the first record 

 of encystment undoubtedly belongs to Nitzsch (1807). Later (1816) this 

 writer records his observations on the encystment of Cercaria ephemera. 

 The review of Nitzsch in Isis describes the process of decaudation and en- 

 cystment as observed by Nitzsch, and shows that this worker expected the 

 cyst to develop the following year. Fantham 's criticism of Nitzsch is essen- 

 tially unjust (1916:12), since Nitzsch considered the cyst to be dormant ands set 

 dead. 



The process of encystment has been described in detail by Thomas in 

 his work on Fasciola hepatica (1883:129). Encystment here conforms to the 

 rapid type described for the monostome C. pellucida. "The tail is sometimes 

 shaken off before encystment begins, but, as a rule, the tail remains in con- 

 nection with the body during the process, and continues to be energetically 



