
JOHNSTON AND BECKWITH—LARVAL TREMATODES 575 
blind tubes arise from the commissure. Olivier (1940, 463), in mentioning the 
well-developed commissures in the eercaria of Apharyngostrigea pipientis, points 
out that “‘the presence or absence of transverse commissures in cerearia simply 
indicates differences in the rate of development of their respective excretory 
systems.’’ The differences in rate of development are, however, group charac- 
teristics and are useful diagnostically; and Dubois (1944, 81) states that the 
diverse genera of Cotylurini and Strigeini represent forms more highly evolved 
than the Diplostomini, since in the former, the anastomosing processes have 
already appeared in the cerearia, while in the latter, they are only developed in 
the metacercaria. 
Finally, Apalemon cereariae are limited in their host range to species of 
Limnaea, Planarbis and Stagnicola, except for the eerearia of A. gracilis, whose 
host is Bithynia; and the metacerearia is a Tetracotyle which forms a thick-walled, 
slightly oval eyst. 
We may point ont that a species of Apatemon, A. intermedius (S. J. Johnston) 
Dubois, has been deseribed from the black swan, Chenopis trata, which is a 
common bird on the Murray swamps. 
Cercarta (FURCOCERCARIA) AMERIANNAB Nn. sp. 
(Fig, 12-18.) 
A new cerearia, C. ameriannae has been obtained on two occasions only, in 
both eases from Amerianna pectorosa collected from the Murray swanips at 
Tailem Bend. It was found in one of 166 Amerianna spp. collected in October, 
1944, and in one of 400 in December, 1946, It is evidently a rare larva in that 
locality since it has not been observed on any other oceasion during our ten-year 
survey of the molluscan parasites of the region. 
The cereariae are emitted mainly during early morning. They are not 
particularly active, spending most of the time suspended in the water with the 
furcae spread at about 180°, They swim in characteristic furcocerearial fashion. 
The length of life was not determined, 
The averages in micra, of ten measurements, based on specimens fixed as 
indicated earlier, followed by the range in brackets, are as follows: length of 
body, 209 (180-229); breadth of body at widest part, 40 (30-49); length 
of tail-stem, 259 (229-295); breadth of tail-stem at widest part, 34 (32-41); 
length of furea, 243 (131-287); breadth of furea at widest part, 20 (16-82); 
length of anterior organ, 45 (37-59); breadth of anterior organ, 29 (27-32); 
length of ventral sucker, 22 (21-27) ; breadth of ventral sucker, 22 (19-27), 
In life, the body when greatly contracted (as in specimen drawn in fig. 12), 
