A TREMATODE LARVA FROM BUCCINUM. 515 
An intermediate host has not yet been identified, but from the struc- 
ture of the cercaria, which is able to modify the posterior end of its body 
as a sucker-like organ, it is probable that the intermediate host is an 
actively swimming animal, as in all probability the sucker is used by the 
cercaria for fixing the hind end of its body whilst the free part waves 
about in order to catch a host. The stylet on the head and its glands 
opening beside it show that the cercaria bores into its host. 
STRUCTURE OF SPOROCYST AND CERCARIA. 
The sporocyst is faintly yellow in colour and measures from 0-5 to 
1mm. in length and is from 2 to 4 times as long as it is broad. Inside 
the sporocyst are germ cells and cercariz in various stages, from | to 8 
in each sporocyst (Fig. 1). 
The full grown cercaria is colourless and transparent, measuring 
0-33 mm. to 0-48 mm. in length according to the extent of contraction 
or expansion (Figs. 2 and 3). The anterior end is rather more rounded 
than the posterior end and usually the greatest width is in front of the 
oral sucker, although when the body is greatly extended the width is 
nearly equal for the whole length, a great amount of extension being 
possible. The oral sucker is a little more than half the width of the 
ventral sucker. Oral sucker 0-06 mm., ventral sucker 0:-10mm. Both 
are well developed and conspicuous. The whole surface of the body is 
covered with minute spines which enlarge towards the posterior end and 
are greatly elongated round that portion which is capable of forming 
the round disc in the middle of which opens the excretory bladder. The 
posterior end can, however, change its shape so that the disc is not always 
present (Fig. 6). 
The oral sucker bears at its anterior end dorsally a thick stylet, 0-015 
mm. long, with a long central and two small lateral points. On each 
side of the spine opening dorsally are situated a pair of long curved ducts 
(Fig. 5) connected with a mass of large gland cells on each side, the 
stylet glands, which occupy the space between the oral and the ventral 
sucker. The oral sucker has a large circular aperture ventrally placed 
near the anterior end of the body which leads to a short pre-pharynx, 
which in the expanded state may be as long as the pharynx but is usually 
much shorter. Then follows a conspicuous muscular pharynx, 0-03 mm. 
long, a thin-walled cesophagus and short intestinal diverticula reaching 
to about the centre of the ventral sucker. In transverse section the 
tubes of the diverticula are seen to be composed of very few cells, some- 
times only two, with large nuclei (Hig. 7). 
The ventral sucker is large and muscular with a somewhat oval centre. 
Immediately behind it and to the sides are the testes, which are well 
