
564 ReEcornS OF THE S.A. MUSEUM 
2-9 p.o. infection. Two of these collections were made at Swan Reach, the cerearia 
being obtained on both occasions. 8, subaqualilis has been found by us in ihe 
River Murray swamps only twice: viz. onee at Lake Alexandrina, when one out 
of 208 snails was infected; and once at Tailem Bend, when one out of 46 snails 
was infected, 
The detailed studies have been made entirely with cercariae from P. tsingi; 
the larvae from the other hosts were identified later as C'_ lessoni by mieroseopical 
examination and measurement. 
The cereariae are emitted mainly during the morning, few appearing in the 
afternoon; the length of life is short, for all are dead within twenty-four hours 
of emission. The cercariae are actively swimming more than half the time; 
when resting they are suspended in the water with the fureae spread at an angle 
of 180°. When few cercariac are present—as in the infectious of P, isingi, a very 
small snail—they tend to collect at the bottom of the tube, i,e. the darkest part; 
in this they resemble C. pseucdoburli Rankin, which is said (Rankin 1939, 88) to 
be negatively phototropic, However, infections of ZL. lessoni, a much larger snail, 
are usually so heavy that a whole test-tube of water may be rendered opaque by 
the numbers of cereariae emitted within a few hours. ‘The larvae swim with 
characteristic fureocerarial movement, tail-first, and, if the tube is shaken, 
vertically upwards. On one occasion two eercariae were observed attached to 
each other and swimming actively; one was eaught on the spines of the other's 
everted ventral sucker. 
For measurement cereariae were fixed by the addition of an equal quantity of 
boiling 10 p.c. formalin to the water in whieh they were swimming. Measurements 
were made with an ocular micrometer, in a water mount, using coverslip pressure 
only sufficient to keep the cereariae in one plane without distortion. The ayerazes 
of measurements of ten cercariae, with the range of measurements in brackets, 
given in micra, are as follows: body length 110 (82-151) ; body breadth at widest 
part 31 (27-45) ; length of tail stem 90 (75-104) ; breadth of tail stem at widest 
part 25 (21-28) ; furea, length 101 (86-113) ; furca, breadth at widest part 19 
(18-21) ; length of anterior organ 23 (19-27); breadth of anterior organ 18 
(16-19); length of ventral sucker 18 (16-19); breadth of ventral sucker 15 
(12-18). 
In life, the shape of the body varies greatly with the state of contraction, 
being sometimes squat and nearly round, and at other times greatly elongate 
(fig. 5). There are no special oral spines. There is a cap of about eight rows of 
fine spines over the anterior organ, followed by about a dozen irregular rows of 
smaller spines, the last at the level of the oesophagus (fig. 1). The rest of the 
body is spineless except for a narrow band of irregularly scattered smal] spines 
