A Study of the Escape of Cercariae from Their Snail Hosts

During the summer of 1920 while studying at the University of Michigan Biological Station at Douglas Lake, Michigan, I undertook a series of experiments on the escape of cercariae from their snail hosts. The object of these experiments was to determine the numbers of cercariae escaping from snails infested with trematodes and the times at which these cercariae made their escape. Four species of cercariae were studied, viz.: (1) Cercaria elephantis Cort (1917), a schistosome cercaria with eyespots from Planorbis trivolvis Say; (2) Cercaria emarginatae Cort (1917), a forked tailed cercaria with a pharynx from Lymnaea enmarginata angulata Sowerby; (3) an undetermined echinostome cercaria from Physa ancillaria parkeri Currier, and (4) an undetermined stylet cercaria from the same host. Most of the studies were made on the escape of Cercaria elephantis from Planorbis trivolvis, the small number of experiments on the other three species of cercariae being carried out near the end of the work to obtain some comparative data. In order to determine which snails were infested with the cercariae the following simple method was used: A collection of about 100 specimens of a given species of snail would be brought into the laboratory late in the afternoon. These snails were then divided into groups of four or five and the groups placed in separate wide mouthed six or eight ounce bottles about one-third full of water. The next morning some of the water from these bottles was examined in a watch glass under a dissecting microscope. If any cercariae had escaped they could be easily seen and the groups containing infested snails determined. These groups were then redivided, only one snail being placed in a bottle, and by further examinations the individual snails infested were determined. Examination of the water from negative groups was repeated late in the afternoon to catch any cercariae which escaped only in the daytime. When a high percentage of snails were infested no preliminary division was made, the snails being immediately placed in separate bottles. With the infested snails isolated it was possible to pour off the water from around them at definite intervals and count the cercariae given off during certain definite periods. A new supply of water was