430 
Marine Cercariae 
(b) Fork-tailed Cercariae. 
Cercaria dichotoma Muller. 
(Lebour 1908 A, p. 13.) 
(PI. XXVI, figs. 1, 2.) 
This cercaria seems to be closely allied in structure to Gyvinophallus 
although the presence of the forked tail seems to warrant its being 
placed in a separate group! It occurs in long irregular sporocysts in 
the little bivalve Scrobicularia tenuis from Fenham Flats in 1 out of 
50. The sporocyst measures about 1 mm. in length, very transparent 
and full of cercariae in all stages. The cercaria measures 0‘20 mm. in 
length with the tail which is slightly shorter than the body and forked 
at its free end. Oral sucker 0’033 mm. across leading directly to 
pharynx, O’Olfi mm. long. Rather long oesophagus, intestinal caeca 
reaching a little way beyond the anterior margin of the ventral sucker. 
Ventral sucker lying well behind the centre of body, 0‘029 mm. across. 
Excretory vesicle lyre-shaped, occupying most of the posterior part of 
body and continued right along tail where it opens at the end of 
each fork. 
Johnstone (1904, p. 107) records it from Cardium edule from the 
Lancashire coast. 
The further life-hi.story is not known. I should say the adult must 
be closely related to Gymnophallus. The form of the intestine, excretory 
vesicle and position and relations of the suckers all indicate a near- 
relative of this genus. The final host is almost certainly a bird. 
(c) The Spelotrema group. 
The adults of this group all live in birds and are very much alike. 
The intermediate host if present is a Crustacean in which the cercaria 
encysts. The cercaria in the cyst can always be recognised as belonging 
to this group as it is very like the adult except that the reproductive 
organs are not fully developed. The typical cercaria, like the adult, is 
tongue-shaped, covered more or less with spines, more pointed anteriorly 
than posteriorly, very much flattened dorso-ventrally, suckers nearly 
equal, prepharynx usually long, pharynx long, oesophagus branching 
into short intestinal caeca which rarely reach beyond the ventral sucker. 
It is generally difficult to make out much more of these cercariae and 
' Odhner (1911, p. 105) describes a fork-tailed cercaria which he believes to be a 
species of Haplocladus. 
