448 LARVAL TREMATODES OF THE NORTHUMBERLAND COAST. 



mussels' foot, the cercaria described here being a good deal 

 smaller. It is also more transparent, and has a curious habit 

 of bending backwards in a U shape with the ventral sucker 

 exserted. E. secundum has 29 head spines arranged in a 

 row round three parts of the head, two at each end being 

 below the others, and these and the spine between them are 

 smaller — long spines 0*025 mm. long|, and in the adult from 

 Lartis ridibundiis they measure 0*050 mm.-o'o53 mm., and 

 the small spines o"026 mm.| My specimens have 29 spines, 

 but although the two end ones are arranged in the same way 

 as in E. secundum, there is not much difference in the size- 

 long spines o*oi9-o"o2 mm., short spines o"oi6 mm. With 

 these exceptions to describe one is to describe the other. The 

 cercaria has a long body with a heart-shaped head bearing 

 spines, and a large oral sucker 0*030 mm. across {E. secundum 

 0*065 mm.), this leads by a thin tube to a pharynx 0*026 mm. 

 long by o"oi6 mm. broad {E. secundum 0*05 by 0*03 mm.), 

 and from this a narrow oesophagus runs down to nearly the 

 centre and bifurcates, each lobe reaching nearly to the 

 posterior end. Just below the bifurcation there is a large 

 ventral sucker 0*046 mm. across (^. secundum 0*095 mm.). 

 Excretory canals begin in two curves near the head and 

 running down the sides, branching frequently, open into an 

 excretory sac with a posterior pore. The canals are filled 

 with clear granules. The tail is simple and very contractile, 

 separating from the body at the slightest touch, and remaining 

 active for some time. Two ducts, one each side of the oral 

 sucker, are to be seen in both cercarise before encysting; they 

 appear to come from glands down the sides, and probably 

 contain the material for the cysts, as they are not to be seen 

 in the encysted forms. In the cercaria from Paludestrina 

 stagnalis there are several small ducts running straight along 

 the anterior sucker and opening anteriorly near the oral 

 aperture (see Plate XI. J) ; these do not disappear in any of 

 the encysted cercariae I have seen. The cercaria moves by 



+ Nicoll, Ann. and Mag. Nat, Hist., Jan., 190G, p. 152. 

 J Nicoll, Ann. and Majr. Nat, Hist., Juno, 1906, p. 516. 



