LAKVAI. TREMATODES OE THE NORTHUMBERLAND COAST. 



453 



mm. It is about four to five times as long as it is broad. 

 At its posterior end is the characteristic and pecuHar tail, 

 broad in the middle, and bearing on each side a long and 

 contractile process which, when extended, is several times the 

 length of the animal. With the help of this tail the larva 

 swims about and enjoys a free existence for a little time in the 

 water. Claparede| found these free swimming forms on the 

 coast of Normandy. The mouth is in the centre of the body, 

 and leads to an oblong intestine. There is a large clear 

 posterior excretory organ, and anteriorly there is the cysto- 

 genous organ with an invagination which Lacaze-Duthiers 

 regarded as" the mouth. The whole surface of the body is 

 covered with small spines arranged in rows, giving it a 

 transversely striated appearance, and it is also minutely 

 striated longitudinally. Mr. Tennant's observations show 

 that this parasite thrives best in brackish water, and that 

 increase of salinity destroys it. Fenham Flats is therefore a 

 favourable locality, for several small streams from the main- 

 land here mix with the sea water. 



EXPLANATION OF PLATES. 



Plate IX. 



A, B, C. — MONOSTOMUM TLAVUM. 



A. Redia. 

 B and C. Cercaria. (s) Posterior suckers. 



D, E. — Cercaria ubiquita. 



D. Sporocyst. 



E. Ce.rcaria. 



F, G. — Cercaria pirum. 



F. Cercaria from cyst. 



G. Encysted cercaria. 



% " Beobachtungen liber Anatoniie mid Entwickelung-sgepcluchte niederer Tliiere, 

 p,n"-estcllt ail der Kiiste Normandie," 1868, 



