W. Nicoll 
327 
pulchella (Rud.). It is a species which appears to be almost ex¬ 
clusively confined to this particular host; it certainly did not occur in 
any other fish at Millport. The best existing description of the form is 
that of Odhner (1902, pp. 497—9, PL 33, fig. 3). It has not hitherto 
been met with in British waters and appears to be entirely absent on the 
East coast. 
The body has an elongated shape, somewhat flattened, but showing 
considerable thickness in the region of the ventral sucker. The colour 
is neutral grey, forming a striking contrast to the rich brown of Heli- 
cometra pulchella , a difference which renders easj' the separation of the 
two species when occurring together in the same host. 
All my specimens are adult and measure T5—2’4 mm. The 
average length is 2 mm. and all the following measurements will refer 
to a specimen of that length. The greatest breadth, at the level of the 
ventral sucker, is 0'6 mm. The breadth diminishes gradually towards 
each end. 
The oral sucker is globular and has a diameter of 02 mm., i.e. -J^th 
of the body length. The ventral sucker is situated at a distance 
of 08 mm. (fths of the body length) from the anterior end. It is 
transversely oval and measures 0'32 x 0'355 mm. It is therefore less 
than twice as large as the oral sucker and about |-th of the body length. 
The prepharynx is very short. The pharynx is large and round, 
measuring 0T3 mm. both in length and breadth. The somewhat 
rectangular shape noted by Odhner was not observed in my specimens, 
except occasionally in the living state. The oesophagus is a trifle 
shorter than the pharynx, having a length of O'll mm. The diverticula 
extend quite to the posterior end of the body. 
The excretory vesicle is a simple one, reaching forward to the level 
of the anterior border of the anterior testis. 
The genital aperture is median and is situated exactly at the level 
of the intestinal bifurcation. The cirrus-pouch is of great length and 
extends back to the level of the ovary. In young specimens it is 
usually not quite as long. Owing to its length, the characteristic club 
shape is not so well marked as in other members of the Allocreadiinae, 
the thickness being almost uniform. It contains a highly convoluted 
vesicula seminalis, a distinct though small pars prostatica, and a long 
slightly convoluted ductus ejaculatorius. In many specimens the cirrus 
was exserted and of great length. In not a few it was found inserted 
into the vagina of the same individual. This appears to be of rather 
frequent occurrence in this genus, for Olsson (1868) figures a specimen 
21—2 
