442 LARVAL TREMATOiDES OF THE NORI'HUMBERLaND COAST. 



already observed in connexion with this species by Siebold*. 

 The eyes are surrounded by many speckles, and are dark and 

 conspicuous. In shape the cercaria is sometimes oval and 

 sometimes shield-shaped, and by contraction becomes perfectly 

 spherical (see Plate IX. C). At the posterior end of the body 

 each side is drawn out slightly above the tail, so that this end 

 is angular instead of rounded. At each of these angles is a 

 peculiar sucker-like structure which does not seem to have 

 been noticed before (see Plate IX. B, s); it is very small with a 

 circular outline, and divided in two by a transverse bar. This 

 reminds one of the small suckers round the posterior end of 

 some of the Pleterocotyleans which are ectoparasitic on the 

 gills of fish {e.g., Octohothrivm melangi, Kuhn, from the gills 

 of a whiting), and is interesting, as it may help to throw light 

 on the origin of the Malacotylea. Two excretory ducts full of 

 granules run down the sides of the body, beginning at the 

 hinder pair of eyes, and unite further down in a clear sac with 

 a posterior opening. The oral sucker is large, and opens at 

 the extreme anterior end ; it leads into a very inconspicuous 

 intestine, bifurcating just below the anterior eye, the lobes 

 extending nearly the whole length of the body. The tail is 

 colourless, long when extended, and very active and con- 

 tractile. It easily drops off, and continues lively for long 

 after its separation from the body. The cercaria measures 

 about o'4o mm. in length with its tail when not much ex- 

 tended, but it may be a good deal longer ; without the tail it 

 is, when at rest, about 0*24 mm. long (Moulinie's measure- 

 mentsf — total length o*8 mm. to i mm., body 0.20-0.50 mm.). 

 This Trematode was found in about 6 per cent, of the 

 individuals of Pahidcstritia stagiialis from Fenham Flats in 

 November, and at Stranraer in August in 20 per cent, of the 

 same mollusk. This is an interesting addition to the British 

 fauna, and specially so because it is now seen to live in a 

 larval state, not only in purely freshwater Gastropods, but 

 also in brackish water species which are covered regularly 



* " Uober die Band- uiid Blasenwilrmer." 

 t " Dc la Reproduction chez Ics Tivmatodcp oiidopavaRites," Mem. de I'lnetitut 

 Genevois, Tom. IIL, 1850. 



