452 
Marine Cercariae 
Littorina littorea at Millport (10 per cent.) in spring, summer and early 
autumn. There is a great reduction in their numbers in autumn and 
winter. The appearance of a periwinkle infected with it is very un¬ 
healthy, the digestive gland being a curious greyish yellow. The gonad 
is destroyed. 
The cercaria is contained in colourless rediae Avith a yellowish 
brown intestine. The redia can be very active and constantly changing 
its shape. The young forms are striated transversely and have no collar 
nor ambulatory processes, they measure about 0T6 mm. in length. 
Older rediae 14 mm. long or longer with thickened mouth, small 
pharynx and conspicuous intestine. Cercaria 0'2 mm. long Avithout its 
tail Avhich may be more than three times the length of the body and 
provided Avith a very thin fin placed dorsally and ventrally for its Avhole 
length. By the aid of this finned tail the cercaria Avhizzes about in 
a most characteristic manner. 
The body is covered with spines. Anteriorly are two dark eye- 
spots one on each side of the oral sucker. Oral sucker 0'35 mm. across 
when at rest but it is constantly being elongated and protiuided like a 
snout. No alimentary canal can be made out. Most of the body is 
occupied by two large masses of gland cells Avith long straight ducts 
running forward and opening by the oral sucker. The excretory vesicle 
is bilobed and oval Avith its long diameter transverse. 
The further life-history is not known. 
REFERENCES. 
Dietz, Eugen (1910). Die Echinostomiden der Vogel. Zool. Jahrb. Supp. 12 
Hft. 3. 
Giard, Alfred (1907). Sur les Trematodes margaritigfenes du Pas-de-Calais. 
Co7nptes rendus des Sci. de la Soc. de Biologie, Vol. LXiii. 
Jameson, H. Lyster (III. 1902). On the Origin of Pearls. Proc. Zool. Soc. 
London, Vol. i. 
Johnstone, Jas. (1904). Internal Parasites and Diseased Conditions of Fishes. 
Rep. Lancs. Sea Fish. Sci. Inv. for 1904. 
Lebour, M. V. (1905). A preliminary Note on a Trematode Parasite in Cardium 
edide. Northumberland Sea Fish. Rep. for 1904. 
- (1906). Notes on Northumbrian Trematodes. Northumberland Sea Fish. 
Rep. for 1905. 
- (1907 a). The Mussel Beds of Northumberland. Northumbe'dand Sea Fish. 
Rep. for 1906. 
- (1907 b). On Three Mollusk-infesting Trematodes. Ann. and Mag. Nat. 
Hist. XIX. 102. 
