LARVAL TREMATODES OF THE NORTHUMBERLAND COAST. 443 



twice a day by the sea. The fact that the ducks in which the 

 adult may be found commonly come to feed on these flats in 

 the winter, makes it quite natural to find the larval forms in 

 the common mollusks of the flats. 



MoNosTOMUM (Cercaria lophocrrca, Filippi). This 

 peculiar cercaria (see Plate X., B, C, D), which appears to 

 be the same as that found by Filippi in Paludina impura, i.e., 

 Bythinia teniacvlafa\, is very common in the liver of Palu- 

 destrina stagnalis from Fenham Flats, and fairly common in 

 these shells from Loch Ryan. It is contained in long colour- 

 less redise (see Plate X. A), which have a small yellowish- 

 brown intestine, and the body alternately swollen and con- 

 stricted. The cercaria is only described very briefly by 

 Filippi, but he mentions the two eyes and the longitudinal 

 membranous crest (" Crete") with which the tail is furnished, 

 and also figures the beginning of a reproductive system which 

 I have not succeeded in observing. He gives no measure- 

 ments. My specimens are small, the cercaria without the tail 

 measuring o"i3 mm. long ; the tail when extended being more 

 than three times the length of the body. Two dark eyes are 

 very conspicuous, and there is a large oral sucker opening at 

 the extreme anterior end. Two large opaque glands occupy 

 most of the body in the more fully developed specimens, and 

 four ducts run from these to open by the side of the mouth. 

 There is a large semilunar excretory sac posteriorly, and 

 directly behind this the tail begins. The body in the oldest 

 specimens is covered with minute spines. The tail is very 

 long and contractile, and bears the peculiar membranous fin 

 or " comb " which is excessively thin and delicate, and seems 

 to be strengthened by set?e ; since when the membrane is 

 destroyed (which is frequently the case) a few setae sometimes 

 remain. This tail is extraordinarily active, and lives for a 

 long time without the body, from which it is easily detached. 

 By aid of this tail the cercaria whizzes about with wonderful 

 agility. In fresh specimens the shape can hardly be seen at 

 all as the creature is incessantly in the most violent motion, 



X Mem. cl. Reale Ace. del Sci. Torino, 1859, p. 201. 



